Hello! I ll try to be brief but give you enough info so that you can search and understand by yourself what you find.
There are two common formats: The VGC format which is doubles with 6 pokemon but you can only pick 4 of them for the actual battle. That format has almost no rules (you can run evasion moves like double team, etc). That is the format the official championships use and is very common for the people that breed and play the game in a console. The other format is the most common in simulators (tho it’s not uncommon on console either). A simulator is a place like pokemon showdown that let’s you build a team of pokemon, while choosing their items, attacks, EVs, IVs, etc to battle other players in a 6v6 best of 1 format. Pokemon Insurgence has its own simulator that includes delta pokemon which I believe is also 6v6.
I will leave the metagame question for the last so that you can first understand the role of smogon in tiers and metagame. Evs are Effort Values and are obtained each time your pokemon gets experience in battle. Each pokemon gives a set amount of EVs when defeated and each pokemon that get any exp. from that fight will get that number of EVs. This EVs give “extra” stat points for your pokemon, about 1 stat point for 4 EVs at lv 100, so that if you train a pokemon against a lot of pikachu (they give EVs for speed) your pokemon will be faster than if you train it against a bunch of mankeys (that give EVs for Attack). So different pokemon will want to get trained against certain types of pokemon. This is only valid until a limit, of course. That limit is 510 EVs, but a stat gets maxed at 255 EVs. That way, you can “max” 2 EVs for a pokemon. IVs are Individual Values and are like the pokemon genes. They go from 0 to 31 for each stat and act as an indicator of the potential for development. It usually translated in 1 stat for 1 IV at lv 100. This means that if 2 pokemon with the same nature and EV training use the same move, but one of them as 30 IV in speed and the other one has 31, the one with 31 is probably gonna have 1 more stat in speed and will attack first. This kinda shows how important EVs and IVs are in competitive as you will always want a pokemon with the best IVs and proper EV training. Of course this only shows one case of a million in which EVs and IVs make the difference. 31 IVs in HP could mean your pokemon can take 3 hits instead of 2 and that gives you an extra turn, or 252 EVs in Attack could mean you can kill some pokemon in one hit while if you had like 100 it would probably 2 hit them, among other scenarios. You can pursue better IVs by breeding or using an IV stone, which maxes the IV for 1 pokemon (IV stones only exist in insurgence, tho). Breeding is the best option for most cases, but IV stones are good for legendaries or pokemon that you are too lazy to breed.
As I said before, the official rules are kinda weird because the VGC 2015 didnt allow most legendaries, while VGC 2016 does. I don’t really know much more about the official championship rules, honestly. Smogon, on the other hand, is a community driven set of rules. This means that it has it’s own rules based on what the community say and vote. The thing is that a lot of people follow Smogon rules because were built by people wanting a competitive 6v6 mode and is always on constant change. Smogon has 2 different set of rules, the first is the usage which is not really a rule but more of a restriction. They divide pokemon in Ubers, OverUsed, UnderUsed, RarelyUsed, etc. The idea behind this is that the most common used pokemon are in OverUser and you can use any pokemon from a lower tier (you can use UU pokemon in OU, for example) but you cannot use a pokemon from a higher tier (like UU in RU). This way, they restrict pokemon based on how many people use them since it is kind of an indicator of how reliable and good it is in general. Gengar is a classic pokemon that almost always gets in OU because of how many people use them and how reliable he is. Some pokemon get attention because of its ability or something new added and get to OU only to lose favor and go back to UU or RU. The tiers are constantly changing based on usage (smogon official place has the exact numbers for each tier, but its not really important for you to try each tier). You can make a team with any pokemon you can (that is not Ubers) and put it into OU since its the most general tier but if you want to start, looking at the most used pokemon can give you an idea of what are people using and how much potential it has. One thing that you must know about tiers in smogon is that they have Ubers which is a tier for banned pokemon in OU, which mean uber pokemon can only be used in uber. This includes a lot of legendaries and some pokemon that carry strategies that are too strong or too hard to counter in OU. The other set of rules Smogon has is the clauses, which are set rules arranged by the community to provide a more competitive environment to the game, some of them are questionable, but most of them provide a metagame that is not very much based on luck. Some examples are the evasion clause, which wont allow any pokemon to carry moves that raise evasion; the sleep clause, which makes it so that only one pokemon can be put to sleep at any given time (to avoid spore or hypnosis spam); or the moody clause that won’t let any pokemon to carry the ability moody (which raises a random stat two times and lowers a random stat 1 time at the end of each turn). This are meant to keep players from abusing what they call not competitive strategies, like going double team on a pokemon until everything has 33.33% of hitting it and depend on luck to get a hit and win. That is also why most people like this rules, so its a lot of people following these rules. Also smogon has a strategydex for each pokemon so that you can check some nice moveset and EV spreads for them.
As for the metagame, each tier has its most dominant metagame since it depends on which pokemon are allowed to be used in the tier, but for OU there is not really a dominant meta, you have to make your team either HyperOffensive and go for a lot of fast and hard hitting pokemon (it has variants like bulkyoffensive, but you have to read about it, its just too long to type here while there are some good teambuilding guides), Balanced and go for pivots, sweepers and wallbreakers, or stall and go for tanks that look to wear down the opponent team with different indirect damage. I recomment you look at Smogon’s teambuilding guide, which, even if you dont understand everything, can help you understand most common teams and how to fight with and against them.
Good luck!