Arbitrator right now is a very small project and as such doesn't have very extensive documentation helping new contributors to get started yet.
The best way right now is probably to hop on gitter and introduce yourself and tell me what you'd like to work on. I will do my best to help, but I will probably not accept any and all modifications to the bot. There are also a number of open issues that can be worked on at anny given time, please have a look at those as well.
If you have never contributed to a Github project before, see this guide on how to create a pull request.