Hmm, I've never had a chihuahua before, but I would guess that because of their diminutive size, you should make sure to be extra careful with leads, etc. And also I would guide the dog with food when teaching basic commands so you don't have do it physically and accidentally hurt your pup. (Like, instead of pushing on their rump, hold the treat above their nose so they sit down).
Like buttercup132 said, what's your dog's name?