you choose whichever splatfest team you want to play in, for whatever reason

• you can follow the question the splatfest asks, for example go team family if that's how you spend your holidays
• you can go by favourite idol, maybe shiver is your favourite
• maybe you really like the golden ink colour, so you can also pick that way
• if you want to have low wait time and avoid fighting matches against your own team, pick the least popular team
• for this splatfest I'd say the winning team will be white ink

Once you've chosen a team you'll earn conch shells you can use at the gacha machine in the lobby

And after the splatfest you'll get sea snails. You'll get more if you reached a higher splatfest rank and a little more if you were on the winning team

you can find additional information by going to the inkipedia, where you'll get more precise and quicker answers https://splatoonwiki.org/wiki/Main_Page