I don't want to be rude at all, but the suggestion of "contributing to an OSS project" makes a lot of sense if you already had to work with/use OSS software. Because, if you had to use one of these projects, you would probably already understand the most important social aspect of it. Coding, IMHO, is just secondary (and is not necessary at all).
Thus my suggestion would be: if you never dealt with an OSS project before, try to find some OSS software which you genuinely like and try to use it (and well) and follow its development. Once you did, you will certainly know how to contribute. There's nothing more to it.
If you are already familiar with OSS, but so far never found anything "interesting", the best thing to do IMHO would be starting your own. Release something that you did and that you would like others to use.
Most importantly, do all of that for fun. Don't do that because you have to and because they said "it would be helpful". Helpful for what? Coding style\quality varies wildly, as is the community around the project itself.
The biggest difference between a "professional" and OSS project is exactly this: people work on OSS project for many reasons, but it's mostly for fun or passion. Some projects strive for quality, some for functionality, and some just solve an "itch" somebody had. Understanding the social aspect, again, is the biggest differentiating factor.
There's no point in contributing to OSS unless:
1) you released something that you like to maintain 2) maintain something somebody else released 3) fix an itch you have 4) having fun coding (or any other activity around said project)