All you have to do to be a Beta Tester is to have enthusiasm, register and start.
Register Betatester
Start by filling out your profile. You will have access to different projects depending on your devices. Remember that a project may be geared to testing with a specific device, and if you have not registered it in your profile you will not be able to access.
To raise your level and win as many badges and points as possible on the way you have to follow the Good Beta Tester guide.