To get started contributing to MIT App Inventor, you should first get a development copy of the software up and running on your machine. Follow the instructions at
https://github.com/mit-cml/appinventor-sources to get started. Once you have a local copy running, you can take a look at the issues list on the repo. Issues marked "help wanted" are usually a bit easier/more well defined as starting projects. Leave us a comment so we know you are working on it.