all you do is press one of the c buttons (left or right i can't remember) and choose the partner you wish. you can also switch in battle by selecting the partner icon on your partner move list.
All nessesary partners come in do time, but here's one more, ms. meows. Here's how you get her, go to the trouble center in east rougepourt after awhile, there will be a trouble from, go to where she...
The sidekicks are, in the order you get them: Goombario the Goomba Kooper the Koopa Troopa Bombette the Bob-omb Parakarry the Koopa Paratroopa Bow the Boo Watt the Hothead Sushi the Cheep-Cheep...
On the field, press the... Right C button, I believe? That or the left. One is partners, one is items. In battle, go to the tactics menu, which is marked by a red flag, and you can access the partner...