Its roughly 1500 games to get to rank 100 with a 50% win rate. Its best to have other reasons to play pvp, where legendary armor is more of a long term commitment reward. My suggestion is just keep playing the classes you like and try to keep having fun. If you try to get really competitive and worry about win rate (since you get more pvp rank when you win) you will likely just become frustrated & toxic and may end up quitting before you reach rank 100.
When starting out i would recommend one of the meta builds, you can look them up on metabattle or godsofpvp. If you like playing thief then keep playing it. It takes a bit of practice but once you start learning combos and map rotations it gets even more fun. You can play off meta if you really want to, but it is much more difficult and it takes a lot more game knowledge to be effective with them.