Welcome to our community and thank you for your question.
In general, it's possible for you to marry no matter what your legal status is. You will have to get in contact with Standesamt (registry office) either in the city where you are living or in your fiancé's city (probably it's more convenient to do it in your city). They will give you a list of documents that you need to provide.
To learn more about the marriage procedure in Germany check out the article on our Wefugees blog that covers some of the most important questions (scroll down to find the English version):
If anything remains unclear feel free to ask further questions.