You can try to trip an opponent as an unarmed melee attack. You can only trip an opponent who is one size category larger than you, the same size, or smaller.
Making a Trip Attack
Avoiding Attacks of Opportunity
If you have the Improved Trip feat, or if you are tripping with a weapon (see below), you don’t provoke an attack of opportunity for making a trip attack.
So its an unarmed melee attack. Can I use my foot? I mean, I've got a morningstar in one hand, and a shield in the other. I've got Improved Trip. I can't use the morningstar to trip, i know that. But can I use my foot? I think I can. Just lookin for another opinion.