RSA encryption project

I'm doing a project for school in which there's an option to encrypt and decrypt a text file with RSA method. I have a few questions about it: I read in some places that it's only efficient for short messages, so is it possible to somehow encrypt a long message (perhaps an article) using RSA? or do I need to use another method (if yes so which is the most recommended)?

I read the theory about RSA but I don't really know how to implement it in C# (I was told that I should not use the RSAcryptoServiceProvider of C#), so I need to make all the variables BigInteger, but how to find a large prime number in C#? If some one has a guide/tutorial/summarized algorithm (not a ready code) It would be very helpful.

thanks