The simplest way to contribute is by reporting bugs that you may have noticed or requesting features you are interested in seeing built.
0x Tracker uses a popular open source platform called GitHub for managing issues. If you are a non-developer then the best place to submit issues will be in the client project. If you are a developer and are reporting a project specific issue then please post it on the relevant project.
Be sure to check whether your issue has not already been raised by someone else. If the issue has already been raised then feel free to signal your interest through reactions or contribute additional thoughts in the comments.