Haha to be honest I don't really follow soccer at all, I just know the main clubs for the Premier League.
Usually my team selection is based on a couple things - if I decide it's from England, I pick the team I recognize most since someone on these forums made "baseball-ized" versions of the all the logos already. Beyond that I just find the easiest combination of high-ranking clubs on the Football Database rankings that I don't have added yet or find teams from new countries or find clubs with logos that don't require much photoshopping to remove soccer balls or whatever. Goal is to eventually have every country represented so I'm sure Leeds will get in before the Vatican gets a squad