Best way to start out is to check
BC's guides which contains a whole lot of stuff to start out on. However that'll probably confuse you and you'll want to start out with the basics, which is getting your team rated where people can explain how to fix your team/make it competitively viable, which can be done in the Competitive Team Help subforum of BC, then afterwards you can re-read over the guides and it'll make a bit more sense. Other than that, the best way to start out is to practice. You can come along to PC's battle server right here:
https://play.pokemonshowdown.com/~~cbc.pokecommunity.com:8000/lobby also to test out teams. If you're confused on how to test teams and stuff then feel free to ask the chat, one of the users with a symbol, +/%/~ will help you out. :3 But yeah, once you've got the basics down, just practice a lot until you're at a level that you're confident in. It also depends on if you're playing wifi or simulator competitive. Most of us here play simulator competitive, so you'll want to get used to Smogon's tiers, while for wifi you'll well... it's really a combination of things that work well in doubles really. So a lot of OU/UU/some RU Pokemon fit in great with doubles, whereas Smogon's tiers tends to be a bit different and usually the Pokemon used tend to be the ones within the list for that tier rather than ones from lower tiers. So for example, something like Skuntank really won't see much use in the higher tiers, or even really RU as much because there's other Pokemon in higher tiers that fit the role so it isn't necessary. Also if you were to play by Smogon's tiering, you should keep in mind that things like Blaziken are classified as Uber, which p much means a lot stronger than OU mons, and dominates/over-centralises the metagame to the extent where you need about 4 or 5 team slots to counter one Pokemon, or it's just way too strong for OU. While the two are different, sometimes strategies and things that are used on Smogon (as well as players) are found in world championships for wifi competitive, so the two can intertwine.
Sorry if any of that was confusing! But good luck getting into competitive, it's really rewarding/fun when you do. :)