Take a look at our issue tracker over on GitHub. There are usually a number of open issues that need doing. Some will be very complex and require deep insight into how our systems work, and these are often handled by core developers. Others will be simple, maybe even trivial, and such issues tend to be left alone precisely because they make very good first-timer contributions. Your first job is to find one of these.
Next, you'll want to talk to us about how you plan to go about solving it. If you can get a staff member to endorse your idea for a solution, the chances are pretty good that your solution will be merged into our systems once you submit your merge request.
As soon as your solution is merged, we will award you with the Contributor role, which grants access to our internal dev channels on the server. This allows you to tackle more advanced issues with our help, and even come up with issues of your own invention to add to our tracker.