The only way someone else can get into your Moshi Monsters account is if they know your password. That is why it is really important to keep your password secret and make sure that it is not easy to guess.
To change your password, just visit this link and follow the instructions:
And here is a great website to help you choose a good password:
Ask your parent or guardian to remember your new password for you, or write it down in a super-safe place.