Keep baths to a minimum, they're only required if the dog gets dirty (or stinky). Bathing too often will dry the skin. Be careful not to get any water in the ears (ears can be gently cleaned with a damp cloth but don't go too far down) and use only dog shampoo Remember Chihuahuas don't like the cold so dry him off well.

Nails
Get puppy used to having its' nails clipped as soon as possible. Even if pup doesn't need a nail clipping go through the motions everyday, that way when he's older he will be less likely to put up a fuss.

Use nail clippers that are especially made for dogs, they come in several designs but the ones that sort of resemble pruners are easier to use, at least that's my opinion. Be careful not to cut too close because if you do puppy may never let you come near his tooties again.

Feeding
Stick to the diet poochie has been accustomed to and gradually change it to what is more preferable to you. Use a name brand quality dog food. Puppies require several feedings daily (3 should be fine), by they time they reach 6 months it should be down to 2 and at one year of age they should be fed 1-2 times daily. Most dog foods have a chart or directions on the amount to feed, follow it and adjust according to your puppy's or dog's needs.

Whether you use canned or dry is up to you although some recommend canned as being better. Some feed a combination of both dry and canned together or keep dry out throughout the day sort of as a snack for poochie to munch on and the canned as the main meal. If that's the case reduce the amount at meal time. Be sure water is available at all times.