I need to know how much you know about competitive before trying to help you. Seems you want to know all in all what the tiers are?
Or just looking for advice for teams and such?
I'm not that pro at it, but I can try and help a fellow user ;)
Okay, so we start from scratch. Here are the tiers:
OU
OU is short for "overused". This tier is the standard battling tier for the pokémon used the most. 5th gen brought a lot of new abilities and strong pokémon, so OU has become a very hard-hitting metagame.
There is also "weather teams". These teams utilize Politoeds DW ability Drizzle and Ninetales DW ability Drought and of course the good old sand stream and build a team around it.
OU can give you a lot of trouble if you are new, but you just need to find someone who'll help you build a team and then you play.
I promise you'll learn a lot really fast, because most OU Pokémon are predictable if you know what they are meant to do.
UU
UU is short for "Underused". Here you'll find many, many different pokémon not as used in OU but still used from time to time.
I haven't played in this tier too much, so I'm not so used to it, but I've heard that in this tier (contrary to OU) is quite unpredicdable. The pokémon in this tier can often do a variety of things, whereas OU pokémon often do one thing and are some of the best at it.
BL
BL is short for "BorderLine". Pokémon that are underused but have too high stats and are therefore banned to this tier.
I don't know much about it though.
Uber
Here roam the legends! Mewtwo, Weather trio, Zekrom, Reshiram etc.
All the strongest pokémon, even some non-legendary pokémon that are banned from OU.
You'll often find pokémon accomplishing things you wouldn't expect.
There are also the RU and NU (Rarely and Never used) tiers, but I barely know anything about that.
But remember: practice makes perfect ;)