How do I become a beta tester?
Make sure to read the warnings in the beta forum first.
Namely the first (sticky) post at the top of the beta forum: *IMPORTANT* Beta Testing Information/Warnings
Also important is the second sticky post: Ten Helpful Tips for Beta Testers
Is it so that if I have more than one computer and a tablet, they all will have beta versions and I don't have any safe place any more?
No - you can set the update channel differently on different computers and so run beta on one and stable on another.
Regarding tablets - if you are referring to the mobile platforms (Android or iOS) they are handled differently.