designing my first robot

I am hoping to build a featherweight but i have suddenly realized that i don't know what i am doing. I have built a few small robots out of stuff i found lying around but i have never built a "proper" robot. What batteries do people tend to use on featherweight robots?

My advice would be to read back through the featherweight build diaries as most of your questions will be answered there. Plus there's some really helpful photos too