Just some 3ds Pokemon game questions for a beginner :) /with Spelunky GA/

Try a website called Celebi it has anything pokemons related on there.

www.serebii.net is probably the best place for info. I used to be really into pokemon. You can get more pokemon in Omega Ruby after completing the 'main quest' than are in the basic pokedex (as in most games) but you will need to trade or get other games to get all the pokemon. You will need far more than 2 games to get all Pokemon without trading - trading is by far your best option. I can help you with most pokemon if you want if I dig out my 3DS that I haven't used in a while...

I'm pretty sure you get a national dex post game where it has all 700+ Pokemon. A bit spoilery but... You can also catch some of the legendaries from previous games post game after acquiring the flute that allows you to soar on Latios/Latias. and there are mirage islands where you can find older gen Pokemon also there is the wonder trade feature which can be hit or miss where you pick a Pokemon to trade and receive another random Pokemon from someone else. like the others said, serebii and bulbapedia would have the most info. and if youre looking for a community, the 3ds or pokemon subreddits have a dedicated bunch.

I also use serebii for most pkmn game related thing I want to know. I'm just waiting for the new game to come out later this year.

For getting more pokemon, I recommend the demo.
http://www.serebii.net/omegarubyalphasapphire/demo.shtml

For all the games there is a regional and a national Pokédex (excluding the first games). Once the Pokemon league has been defeated you will unlock the national Dex which allows you to own all the Pokemon in that game and all those that came before it.

I think the combination of omega Ruby and either x or y will allow you to obtain all Pokemon including legendaries and starters

How about Bulbapedia. I always use it.

There should be plenty of guides on Youtube that show how to catch certain Pokemon.

Soo, just chiming in here because I like to consider myself somewhat of a collector...

You CANNOT catch all Pokémon IN game without special event codes. https://www.steamgifts.com/discussion/Qysoi/gonepokemon-xyoras-mew for example. There is no "Mew" in game to be caught, it's considered a Mythical Pokémon and MOST of those require some kind of special event, either signing up for Pokémon Bank, going to a local Gamestop/Toys-R-Us, going to a World Battle/TCG event, etc. So yeah, you can catch a LOT of the Pokémon with a combination of X/Y and OR/AS but you cannot catch them all sadly.

EDIT: Though, through the wonders of the Global Trade System you could potentially trade and receive every Pokémon in game. You would only need about 700 shiny mews to do it.