One of the best tools for maintaining a code base (and your sanity) is version control systems. Tools like Git make it easier to build out a new feature in an application, and collaborate with other coders. Essentially you can think of it as sort of a way of keeping track of drafts of code. These tools keeps track of who did what, when, and in what order. They also allow for merging code written by different developers.

Git is a popular version control system. It can be very confusing to use the first time you try it because it features a lot of its own commands. You can do it!

Another more simple overview:

Even more resources: https://help.github.com/articles/good-resources-for-learning-git-and-github/