There are several ways to become a freelancer, depending on which kind of business you are going to start.
Some jobs are classified as Freier Beruf, which basically means that you only have to pay taxes for what you earn after registering at the local Finanzamt (tax office). This includes doctors, lawyers, journalists, translators, researchers and many more.
As for IT, there has been a special ruling, which says that "trivial software" does not fit into this category. This means that you have to prove that whatever software you're developing is adequately written, i.e. you have some kind of software engineering degree or equivalent qualification.
In any case, the Finanzamt will tell you whether or not your job counts as a freiberufliche Tätigkeit, which is what freelancing in Germany is called.
If for some reason your software is deemed so trivial that you're not qualifying as a freelancer, you will have to register a company in some form or another. The easiest way to do this would be to register a Kleingewerbe.
This is a regular company which does not have revenue exceeding 17.500 euros a year (last I checked). It requires minimal paperwork each year, summarizing what income and expenses were made (EÜR).
If you're earning more than that, you have to do proper accounting. I don't think there is any more to it than that, unless you're looking to create a limited company in form of a GmbH. I doubt this is what you're going for.
In any case, best idea would be to at least talk to an account / tax advisor once to check what is the best course of action for you and also how to make the most of your situation.