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