Okay, i want to start building a team for competitive wifi battles and the like, bute whenever I go to that smogon website I just get so confused, there are different tiers can you only use the pokemon of the tier or can you use a lower tier pokemon in a higher tier if you wanted to (can you use a UU in OU i mean)

I always hear no legends rule (which I really like) but whenver i try to look up matches on youtube to get an idea for how the game works there are TONS of legendaries

What are some basics i need to know before i start