You start off as a Babbivore. You are also hungry. Attack grass to rip it out of the ground and eat it. Attack trees to knock fruits out (maybe) and eat them. I wouldn't advise going after the other creatures at the very beginning of the game.
Eat enough stuff, and you grow into a Midbabbivore. Which is basically a bigger blob. You feel a little stronger than you were when you were a Babbivore. But you're still hungry. Eat more.
After that, you become a Bigbabbivore. You have leg-stumps but don't quite know how to use them. You're still hungry. But CAN YOU FEEL THE POWER GROWING WITHIN YOU? Eat more.