When can you start?
Most companies are looking to fill their open positions as soon as they find the right candidate. The interviewer is also looking to learn if you are interviewing with other companies.
If you can start immediately, say so. If you need to offer your current job a certain amount of notice or are waiting for other interviews, let the interviewer know how much time you would need. Bear in mind that if the company's need is immediate, a start date that is too far out may result in not getting a job you otherwise would have been selected for.
"I am eager to start as soon as possible. I have applied for a couple of other positions with other companies but this is my first choice. If you need me tomorrow, I can be here tomorrow."
"While I would love to join your team tomorrow, I am working on a project with my current employer and I would like to wrap it up or get them to a good place with it. I feel it would be unprofessional for me to leave without notice. I would like to give them two weeks' notice, but I am ready to sign the contract today."
