Smogon's DP competitive battling guide
While outdated by a generation, it's still good information on the basics of competitive battling.
If you want to gain some experience, you can watch battles on Showdown, and see what other people do when making teams.
Basic things to consider:
- synergy (how well 2 Pokemon work together to counter their weaknesses)
~ e.g. Scizor/ Rotom-Wash, Celebi/ Heatran
= these Pokemon are also referred to as 'core'
~ synergy can also be used to refer to how well your team holds together
- type resistances
- team support
~ i.e. Pokemon that can use moves like rapid spin, heal bell, wish, stealth rock, reflect, light screen etc.
- tier specifics
- Pokemon that pose threats
~ e.g., if you're making a team and have 2 or more Fighting types, your main concern would be Flying and Psychic types
You can also look on the RMT and see how other people make teams; some good teams are accompanied by the thought process that went into making them.
There's also the mentor system on this site: link, where you can request for someone to tutor you in specific or general aspects of battling.