Do you have a GitHub account? I would suggest you start with making an account, and installing git first.
If you are on windows:
https://windows.github.com/Also as was previously mentioned, Visual Studio for editing code:
https://www.visualstudio.com/It is possible to develop without these tools, but why?
Linux has similar equivalents, (eg. the command-line git and mono-develop).
Once you've got that all set up, you can download the source for the ZK Infrastructure project:
https://github.com/ZeroK-RTS/Zero-K-Infrastructure.git(try reading some git tutorials if you're unfamiliar how this works)
A list of known, outstanding issues are here:
https://github.com/ZeroK-RTS/Zero-K-Infrastructure/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3AopenBut you don't have to limit yourself to those either. There are several things that are not documented, but still need improving. If you come across something that looks bad, or doesn't work, feel free to figure out why and fix it.