As the other commenters have said, most a professional historians' time is spent either teaching or researching (writing scholarly articles and books) as part of their work at a university or (in Continental Europe) a specialised research institute. Depending on a wide range of factors, a historian will teach or research or split their time between both. There are a minority of historians who work at historical sites, libraries or musuems, usually focusing on public education and outreach, but occasionally in research positions too. The path to becoming a professional historian is a long one and the pay is relatively low; anyone with the intelligence, determination and skill to become a professional historian could indubitably make a lot more money doing many other things.

As for how you become a historian, the first step is getting a good undergraduate degree in history or related disciplines (in Italy, where you've indicated in now-removed comments you're based, I've heard anecdotally that history is often combined with archival studies, manuscript studies and palaeography at undergrad level.) Because of this, at your age you should focus on getting your school-work at a level to gain admittance to the best university course possible. Within this, you should focus your attention on history (obviously) and languages - based on my limited knowledge of the Italian education system, you're pretty well set with Italian, English and probably a grounding Latin and a third modern European language too. Finally, read every history book you can get your hands on outside of class (see recommendations below).

After your undergrad, the real work starts. Be very careful which advice you follow here - the graduate education system and the employment markets for historians are totally different in North America from the UK and different again in continental Europe. Usually you'll need to complete a Master's degree (usually 1 year in UK, 2 years in continental Europe, optional in North America) and a PhD (usually 3-4 years in EU/UK, 5-10 years in North America). After finishing your PhD, most people work for several years on short-term, insecure contracts (either as teachers or researchers) before a lucky few manage to find a permanent position. It's a long, hard, low-paid road. I should also mention that virtually everywhere, luck is just as much a factor in success as merit is; in many places (particularly in Continental Europe and even more so in Italy) politics and nepotism play a very big role in appointments to jobs and even PhD spots. You'll always need to have a back-up plan.

[This stuff is all, obviously, anecdotal, but I've heard these stories enough times from different sources that I put a good deal of credibility in them. Obviously, your results may vary.]

However, all of that stuff is decades ahead of you at the moment. As I said above, the most important thing right now is to read as much history as you can and find what you enjoy. Since you mentioned the early middle ages, you can't go wrong starting with Chris Wickham's The Inheritance of Rome. Wickham has written lots on Italy too, including several works in Italian, although they are more advanced and aimed at a specialist audience. His Italy in the Early Middle Ages is still the standard textbook on early Italy, despite being 35 years old, while Medieval Rome (published in Italian as Roma Medievale) is an excellent example of a top-quality historical research. Outside of Wickham, Julia Smith's Europe After Rome is an excellent alternative take on the early middle ages, focusing on culture; MacLean, Costambeys and Innes' The Carolingian World is the best introduction to everyone's favorite medieval Roman-revival and Peter Sarris' Empire's of Faith is a nice Mediterranean-focused survey which covers Byzantium and the Islamic World too. In Italian, Giovanni Tobacco is the grand old man of Italian medieval historians and an excellent starting point (I assume he's even better in the original - I've only read translations), while Laurent Feller and Francois Menant are two prominent French historians who've published quite a bit in Italian too. If you read French or German there's even more, but that should keep you busy for now!

I'm currently in grad school for history and the professor I am TAing for this semester has a REALLY good write up on his website:

http://www.centraleuropeanobserver.com/career-guide

He has a lot of good resources on there too.

I'm historian. :-)

1. You'll need to go to grad school and get a PhD, especially if you want to teach at the University level (or even at a private high school). It's never too early to plan out your undergrad, where you want to go to both undergraduate and grad school, and which universities have the best programs. Early medieval historians are rare, so do some research and find someone you want to work with and a program that will give you research experience at the undergrad level. That will make a big difference in where you get into to grad school, future job prospects, etc.

2. We aren't paid much, even as professors. 45,000 to 65,000 for a starting assistant professor is a good ballpark.

3. We teach, write, and answer e-mails. Some research is thrown in there too. :-)

4. Specializing in the early middle ages and the training you'll need: You will need to learn several languages, including Latin and the language of your field of choice. Start learning these languages in undergrad and plan on studying abroad if you are in the US. You have to test out of at least two languages in grad school and you'll be doing most of your research in Latin.

5. Get involved with history organizations early on. The American Historical Association is a good place to start and the professional organization for medievalists is the Medieval Academy of America. There is also the International Medieval Congress at Kalamazoo which is way less formal and a good place to start presenting papers. Most of these organizations have funding for grad students and undergrads to attend.

6. Books. There are so many here but start with Christopher Wickham's Framing the Middle Ages and scan the bibliography and notes for other books. Heads-up: lots of academics history is dry but it can also be really fun to read.

Good luck! :-)

Practice good writing techniques now. It's far easier to struggle now and have a good or acceptable writing style now than it is to unlearn your bad habits as you get older. In History at least, at all times, I am hammered about the usage of the passive voice. If you don't know what that is, look it up now and perfect the use of employing the active voice in your writing. Learn the proper grammatical structure now so you're not tearing your eyeballs out in the future.

Learn how to construct an argument, how to collect evidence and utilize it, recognize and avoid fallacies. Lastly, IMO, learn to write poetically and beautifully. You can improve your writing stylistically, but if you don't have the solid foundation from which to proceed you will slow yourself down unnecessarily.

This is the technical side of becoming an academic. The others have mostly alluded to the social and practical aspects. I'd add one thing, find specifically what in the Middle Ages interests you, and find a school with a scholar who does work similar to that. In so doing, you're setting yourself well for both undergrad and your MA. Expect to attend a different university for your PhD, and in that regard, again, be on the lookout for top tier programs or scholars in your field of interest.

Wish you all the best. You sound a lot like I did when I was your age. I'm not there yet (writing the dissertation currently) and I've been a bit soured by my own experience, but I would never attempt to dissuade someone from pursuing their dream. It hasn't turned out like I had planned, but I wouldn't change this for the world. Good luck!