Keep a close watch at your creature's health meter. If it almost empty - go search for some meat balls (if you are a carnivore) or vegetables (if you are an ominvre) - you will not only repair your health, but also stimulate your creature's developement.
ALWAYS keep away from creatures bigger than you are. On higher difficulty level dealing with them means suicide, on lower - terminal wounds. Run away from them - remember that.
You can defend in many ways. Notice, that during your creature's developement additional option would be added to the editor - thorns, poisoning or electrocuting your enemies. Always try to make set these things in a way, that would harm your foes.