Getting started in Support is a comfort level thing. You help where you’re most comfortable and because of that, there’s no ‘one size fits all’ reply to the simple question of “How do I get started?”
We can, however, give you some general pointers.
Follow this blog! We talk about stuff here, and try to post the important bits regularly.
If you want to help in the forums, join the #forums on Slack and WP-Forums email list. Even if you’re not a moderator, we discuss problems and solutions there. It’s NOT a support list, and it’s not for complaining about moderators. It’s for volunteers helping volunteers help others.
Like IRC? We have a channel on Freenode called #wordpress and we’re always welcoming more.
If you’re really into documentation, rather than helping people one-on-one, go to Make Docs and jump in. There’s a lot of crossover between the two groups since many of us get tired of answering the same question and jump in.