If it is a random project, then you are pretty much lost. So, Do not contribute to them. Think of a project you use and where you would like to see a few features. Pick it up, subscribe to the issues for a few days and get some familiarity. After that, compile it and put a few debug points.
Also, write to the developer(s) of the project that you are looking for something to work on. If they suggest something, work on it.
That said, I think Apache Foundation projects are pretty well documented. Try them.
This is all advice from a person who never contributed to OSS.