It's not a good idea to have strangers pick a project for you. You should choose a project that you like and care about. That way it's much easier to sustain interest in it for the long run. Maybe something you already use, and for which you have some ideas for improvements.
An idea of what, exactly? The way to get involved in a project is to get involved with it :-) Big projects have sections on their web site that explain how to get started with contributing. For smaller ones you can contact the developer, or follow the mailing list. If the project is on Github you can offer pull requests with improvements you made.
A good start might be to engage with the current project contributors. Perhaps comment on an open issue that you think sounds like something you could help with? Perhaps the project owners have a discussion group or forum you could use to strike up some conversation with them? Making contact with the existing active contributors might result in them pointing you at something they think you could help with, or if you're really really lucky they might invite you to help develop something as a joint effort.
As Tim M says, the first task is to choose a project you want to contribute to. It's really important to pick something you use or at least have an interest in. Doing otherwise can make it really difficult to maintain the motivation to persevere with it.