So his name is " Cassiodorus" (Flavius Magnus Aurelius Cassiodorus Senator) born around 485. What should I be carefull about? Im writing something like this for the first time!
Should I just go ahed and explain that he was bron into a rich family and that he has been working as a Abbot for a time before he went into politics?
When you're writing about a person, it's important not to just write a biography. There are tons of those around and when doing history research, it's more important to find one aspect of the person and focus on that particular thing. With Cassiodorus, you have a lot to work with. Use resources like Google Scholar and JSTOR, as well as library books and journals to find more information.
Cassiodorus was a great contributor to education, writing, and preservation of Classical works, as well as being a statesman. You can consult some of the more primary resources, such as this: http://www.gutenberg.org/files/18590/18590-h/18590-h.htm and just find something about him that sings to you. Delve into it deeply and tease it out as the main body of your research. You can and should add some biographical information, but keep it brief and make the bulk of your paper about a more narrow aspect of his philosophy, life, or contribution to history.
Also be sure to use citations, if required by your teacher, and consult both primary and secondary sources. If you're reading a book about Cassiodorus, you can check the author's citations to lead you to the sources he used to get his information and find more details for yourself.