Hi! I would like to learn good habits and all-around general skills to become a better historian.

The first thing to keep in mind is that no-one expects you to be able to get to fully to grips with historical practice this early in your course. Any good History programme worth its salt should be actively seeking to develop your historical skills and competencies, not just your knowledge; that is a process that should continue well into the final year. No one wanders into a university and starts producing publishable work immediately!

With regards to your question about how and why, in your phrasing, the latter is what historians aspire to do. We are not writing to try and judge the events of the past; we are seeking to paint a picture of it as faithfully as we can, one that deepens the understanding our readers have of the events and people we discuss in our work. In order to do that, we try our best to understand the past in an objective and critical fashion; we try to immerse ourselves in the history we write through the fragments of the past we study, rather than look on distantly and pass judgement.

Of course, one of the great debates in historical practice - one you will probably encounter later in your programme - is whether or not that is actually meaningfully possible. We are after-all the products of very different societies to the ones we study; absolute objectivity and immersion are impossible in that context. Nonetheless, we do aspire to it in the interests of developing a richer understanding of the past. We aren't trying to decide who and what were good and bad - we are trying to understand the complexities of people and events, and present our findings to our readers.

That isn't to say that you must necessarily be dispassionate - some fantastic work has come out of historians pursuing the history of their own identities, or the stories of people they are sympathetic to. Similarly, you will inevitably form your own subjective opinions and moral judgements as you go about your work. You're Human, after-all! And there are some moral judgements that are quite unavoidable - it would be pretty difficult to find someone who came away from the study of the Holocaust thinking it was a thoroughly good and decent thing. But that personal moral judgement does not prevent us from understanding why contemporaries responsible for it may have felt that way. We can understand events and people of the past without being sympathetic to them. Similarly, our modern sympathies do not prevent us from understanding that our arguments must be rooted in rigorous and critical research; we must be open to having our initial ideas and judgements challenged by the evidence we collect.

As for our motivations, the "to prevent bad things happening" is a bit of a cliche. I would say there are two main motivations: first, plain and simply, we enjoy it. Many people take great pleasure in learning about the past for its own sake, and many historians cherish the opportunity to bring to life the experiences of others through their work. At the same time however, it is not so much history prevents bad things from happening - it helps us to understand our world today. A part of that is trying to make the future better, but it is also a goal that is worthwhile in and of itself, in my view.

Research

Highlight everything: If you struggle to identify what's useful in a source, make rigorous notes on it. If anything might potentially be significant (a fact, a figure, an argument, an interpretation, something to chase up etc.), highlight it or write it down. This may make it easier for you to identify links and themes, and develop a thesis. With primary sources, always consider who, what, when, where and why. If you find it helpful, write a line or two in your notes about each of those things to look back on.
Avoid the JSTOR effect: Just because it's in a journal don't mean it's good. Don't just type a few keywords and assume whatever pops up must be good. An article from The Journal of Negro Studies in 1942 probably doesn't reflect modern historiography. Be critical when searching for literature and consider how what you find ties into the broader scholarship. Read reviews, not just books.
Consider what research is appropriate: Some courses will absolutely not require independent secondary research. If you're learning from a leading expert on a specialist topic, you can assume the reading list is going to be adequate for the assessment. Always look to the reading list first, and consider whether or not it's necessary to go beyond it to write your paper.
Look for best practice: Consider what kind of sources historians use when tackling topics similar to yours. Your tutors know there are limits to an undergrad essay; they are more interested in seeing you use one or two sources really well than seeing evidence that you've collected dozens (though for some dissertations, the mass research approach is a viable one, especially ones on new ground).

Essays

Your thesis: Every essay needs a thesis. Your thesis is your interpretation of the problem and its proper solution; it is more than your opinion, and it should not simply restate the question. It needs to be specific, systematic and logically defendable. Generally speaking, your thesis begins as your initial response to the question based on your existing knowledge, and is developed and refined (and often fundamentally changed) by your research. It is your solution to the problem based on your critical engagement with the source material. This post I wrote about developing historical questions may be helpful for this. A good thesis needs to be defendable and specific. For example, "After 1865, former slaves had a hard time integrating into the US economy" is not a strong thesis; "After 1865, former slaves had difficulty integrating into the US economy due to a systematic effort by the white planter-class to exclude black people from civil society in an effort to maintain as closely as possible the socio-economic order of slavery" is much more robust. A good thesis requires a point, an argument and evidence to back it up.
Roadmap your essay: Your introduction should offer your reader an insight into where you are going, without giving the whole game away. Outline what your topic is, its significance, how you will approach the question and what the crux of the essay - your thesis - is. Whilst all the juicy details should be left out, nothing after the introduction should be surprising to the reader.
Define the parameters: Show that you understand the question. It is good practice to try and define key terms (either in the introduction or immediately after it), and place the question into its broader historical context. Avoid dictionary definitions - it is much better to take a definition from a well-received historian's work. If your essay will be focusing on a case-study or narrowing the focus of the question (e.g. only talking about Jamaica in a question on the Caribbean), outline that in your introduction and offer an explanation as to why.
Engage with the scholarship: When writing about the work of other historians, it isn't enough to just comment here and there about what has been said. You need to really engage with the scholarship, in a critical fashion. If your reading reveals a debate between historians, don't just select the ones convenient for your argument: address the debate and comment on where your work fits into it. If a particular book strikes you as significant, note that fact (e.g., X says in his seminal work Y that Z happened because of...). Similarly, don't critique for the sake of it, but if you can make a legitimate criticism and back it up, go for it.
Use of evidence: Primary sources are not there to recount what happened or just to flesh out the secondary material; they are the evidence you deploy selectively to support your thesis. You may hear a lot about essays having 'original thought'; students often assume this means writing something brand new, but this is not the case. No-one expects you to be an expert, but your tutors do want to see you coming up with your own ideas and criticisms based on the primary sources, and rooted in an understanding of the secondary literature. Use primary material to build, defend and justify your thesis; your analysis should highlight why the source you have chosen is useful and elaborate on how it contributes to your argument. If there are problems with the source, discuss them and, if possible, alleviate them or explain why they do not diminish its value too much in your case. Try to avoid using primary sources only to go on to say "this demonstrates HistorianX's point that...". HistorianX has already found the material they need - it's your thesis we're trying to prove. Instead, tie your insights into the scholarship - it is your analysis that gives the source authority. Quote selectively and sparingly.
Structure your argument: Your essay should flow logically and systematically. Tie the conclusion of one section into the beginning of the next where possible. Make an argument throughout the work; when addressing debate or different options, do not run through them and then dump a conclusion at the end. There should be evidence of your conclusion throughout any discussion or presentation of evidence. Again, there should be no surprises.
Referencing: if you aren't sure how to properly reference something, check the detailed version of the style guide, rather than your university website. If you're still lost, try if you can to check with the tutor who will be marking your work. If you can't find help, remember that being consistently off is better than being occasionally right - as long as your referencing style is logical, consistent and in the spirit of the format, you should be okay for an undergrad essay.