The best way to contribute is to pull the code from the appropriate develop branch on Github and start hacking on it, to fix a bug or add a feature that you (and hopefully others in the community) think is necessary.
Once you have your code written, please test it thoroughly (and, if for counterparty-lib, please run the test suite via the counterparty-server tests command).
Upon passing all tests, please create a pull request for the change, and submit it to us.
Please note that unless you are proposing to fix a critical bug on master, we only accept pull requests to the develop branch of any given code repository.