The very first thing you'll want to get an understanding of, if you don't already, is EVs, IVs, and natures, which this page goes over:
https://forums.serebii.net/threads/a-basic-guide-to-evs-ivs-and-natures.499840/
If you are completely unfamiliar with competitive battling in doubles, this short introductory article may be of use:
https://www.smogon.com/smog/issue7/vgc
As for team-building, these two things may help you:
https://www.nimbasacitypost.com/2017/03/vgc-2017-teambuilding-guide.html
https://daily.pokecommunity.com/2016/08/21/antis-competitive-battling-tips-team-building/
The first one will likely be more useful to you atm. The second one is written from a singles perspective (ORAS OU specifically) and not a doubles one, and is also intended more for people who've already gotten their feet wet, so it may be better to read it later instead of now, but the some of the basic principles should still apply to some degree.
You'll want to take a look at this when making a team:
https://3ds.pokemon-gl.com/battle/usum/#double
That's the usage statistics for USUM Battle Spot Doubles. It shows the 30ish most commonly used Pokemon in the format, and provides other useful information, such as what moves and items each 'mon commonly uses, what they're often paired with, what commonly KOs them, etc. An important part of battling is understanding the meta so you can prepare for it, and usage stats like these do a lot in that regard.
If you still want some basic but more specific ideas on actually starting a team, I could probably dig up some sample cores or teams for you to copy/make use of (in part or whole). Alternatively, you can just go "I want to make a team with X 'mon on it" and work from there.
And the key thing, above all, is to get plenty of practice. Sometimes experience is the best teacher.
Don't be afraid to ask more questions if you need to, maybe post a RMT sometime once you've actually got the team built.