When are you able to start?

"When are you able to start?"

If you are not working, then you are able to start immediately. But saying immediately sounds too desperate so tell them next Monday or something. If you are currently employed, then you have to show that you are responsible by giving your current employer a two week notice.

"I'll be able to start as early as next Monday."

"I have to give my current employer a two week notice so I could start immediately after that."