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There are three ways, depending on how competitive/autistic you want to be.
1. Pick your six favorite Pokemon. Done!
2. Pick your one favorite Pokemon. Find what types it's weak to, then pick Pokemon you like that are strong against those types. Repeat until you have 6.
3. Pick your one favorite Pokemon. Go on Smogon or some other online meta-analysis page, find what tier it's classified under, then repeat option 2 picking only Pokemon from that tier.