>>17979951This gen seems to like bulky offense, so I'd suggest starting with a good defensive core (mons that work well together countering each others counters/etc; pretty sure there's a thread on smogon that lists good defensive cores) that best fit the mon you want to build around. After that, find complementary mons that help out what's lacking (ex. need more offensive presence, so put in WPolicy Dragonite, etc).
Honestly, it's better to start playing with a team you've put together with some thought, and then playtest it until you can clearly see weak-links and why they're not effective (ex. Forretress loses a lot of momentum on my team and is setup bait in almost every match, I should look for a different spinner, etc).
Getting advice on teams just skips a few playtest matches. In time, you'll learn to predict what's a relevant threat all teams you make should cover, etc.
For weather, start with all the mons that abuse rain. Kingdra, Ludicolo, Kabutops, and Omastar are your premier Swift Swimmers (in that order), Ludicolo and Kingdra can even afford to setup rain themselves. Next are your Thunder/Hurricane abusers (Tornadus-I/T, Thundurus-I/T, etc), Dry Skin/Rain Dish users, and mons that appreciate Fire damage being halved (most all Steels, esp. 4x weaks like Ferro and Scizor) or just appreciate Water damage boost anyways (Rotom-Wash, etc).
Even when developing a themed team, always leave two spots open for mons that'll complement the set you choose (ex. all the rain abusers you choose are weak to Grass, consider a Ferrothorn; all of them depend on setup and lose to Taunt/Encore/Toxic, consider Magic Bounce/Prankster/Magic Guard bulky pivots, etc).
I'd definitely read the BW Rain guides (OU has one, and some other tiers have one, like RU I think) for gen 5 on Smogon for ideas on how to make a rain team.
Honestly, just play more, ask questions, and think about all you do. There's no set guide on team building. Reading RMT threads on Smogon can also help.