Just apply to any that interest you and give great feedback. That means giving the level of feedback the writer needs, testing elements requested, and paying attention to requests writers make (usually posted after version updates).
I like specific and passionate feedback. For example, “I really liked how Heather sticks up for Tom at the meeting but wished I had an opportunity to tell her off.” Or “Great dialogue between Devlin and Eugene but it went on too long and my character felt left out.”
I added some complex code and want people to break it. In the past, players have figured out ways to get additional Skill points by repeating specific options, so I am hoping testers can try to find those holes.
Often you will see an announcement to test official games before release. These are great opportunities to learn what it takes to test and build a resume.
If you want experience, I will invite you, because your post suggests you would take testing seriously. Just PM me if you still want an invite.