Starting Comp?

Currently, you are in the best spot to right now as the current seasons of Highlander and 6s are about to end and in a few months, the next season will start.

What u wanna do right now is go to RGL's website, join the discord and look into the 'looking for player' and 'looking for team' sections. you will have to decide on what form of comp u wanna join. RGL's main page has a good explanation on what forms of comp you can run. Currently there is no prolander or no restriction 6s.

6s is 6vs6 with 2 soldiers, 2 scouts, 1 medic, and 1 demo. The map pool has a lot of 5cp and koth in contrast to Highlander, which is 9vs9 one of each class. The map pool for highlander has more of Attack/Defend and Payload with less koth and no 5cp (as of last season).

Look to join a Newcomer team as it's supposed to be for people who have no competitive experience, you could join an Amateur team as they tend to had a bit more experience but still kinda entry level. You will also need to clarify what class you want to play in either game-mode.

You could try out UGC, but RGL is the current/best way to start TF2 comp.