To be honest, a 10g tank isn't going to be enough room for your goldfish. You really need about 10g per fish. Believe me, they'll be pretty big by the end of summer. A lot of people use something like a kiddie pool, a rubbermaid container or a horse trough to put their goldfish in during the winter.
All you really need to do is use some vinegar or bleach and water to clean a used fish tank. If you used bleach, just dechlorinate well on the last rinse, and leave to dry.