I know you guys don't necessarily have to follow my advice, but in my teams, I generally try to get all three of the starter types in there: Fire, Grass, and Water. They're very practical types to have as you'll most likely need them in a lot of battles. I mean, by having just those three types on your team, you can already counter about 8 different types effectively, many of which are very common types. And that's not counting any primary or secondary types the Pokémon you choose may have (you'd probably want to pick up a Grass Pokémon with another type to counter some of Grass' weaknesses).
Obviously, my starter (or your starter) fills one of those slots. But I heavily suggest picking up two more to fill the other two.