As described in ST and Damage, Normal kicks, punches, and bites all do -1 point of damage per die. This should be computed as full damage using 70% of the character's ST and recorded on the character sheet. Advantages like Sharp Claws and Sharp Teeth that modify the damage type but not amount are also based on this unarmed combat damage.
A punch has a -2 to hit anything below the character's hips. This usually means that punches to another character's legs and feet suffer the hit penalty. As described in Hit Locations, punches roll a 1d6 for hit location when two same sized characters punch at each other from the same height.
There is no penalty to kick at the level of the hips, groin, thighs, shins, or feet or to stomp on opponents that are on the ground or which are much smaller than you. As described in Hit Locations, normal kicks roll a 1d5+5 for hit location when two same sized characters kick at each other from the same height. A high kick is any kick made above the level of the character's hips. It gives a -2 to hit, requires a roll against Brawling, DX, or kicking technique to avoid falling down, and rolls 1d6 for hit location under "standard" conditions.