I want to try out playing competitive on Showdown or something since battle spot is plagued with legendaries and after 2 days of playing i found that the diversity is horrible
But how do i actually build a team? i heard that getting good at battling comes later and building is more important but what do i need?
Let's say, i want to play in OU with Mega Loppuny as my Mega or play in UU with M-Beedril
What else do i need? I'm completely new to these things and know jack shit about meta or things like stall team or hazard and stuff

Thanks for responses, I'm currently skimming through a lot of things at smogon forums to increase my knowledge