I agree with the changes Bron Brooke has made except for the last sentence. It should say “...it sometimes shows its tummy”. The possessive pronoun (its) doesn’t have an apostrophe.

Not sure what you’re looking for, but I’ve made a couple of small changes so that your sentence is perfect! I have a hamster. It eats melon seeds and wheat. It sleeps a lot and when it sleeps, it sometimes shows it’s tummy.

You must use a capital letter at the beginning of a sentence.