We would need more information.
1. Is it a glass tank or a acrylic tank. If it's an acrylic tank, then use a hole saw. If it is glass, use a Dremel type tool. Matt (Wombat2) has drilled glass tank for people before - so PM him.
2. If The size of the hole will depends on the size of the bulkhead. When you purchase the bulkhead, it will tell you what size hole you need.
Minh