Tell the interviewer your availability with respect to a start date for this position, should you be hired.

Should you not have any date in mind, mention that you are available upon providing two weeks notice to your current employer - if employed. Never tell the employer you are available to start immediately if you are working for someone else. This shows a lack of respect toward your employer as you aren't providing adequate minimum notice for him or her to find a suitable replacement, and that you may do the same with this company should you be hired and should you decide to leave at a point in time. Leaving without providing adequate notice may be held against you.

Should you be unemployed at the time of the interview, tell the interviewer that you are available to start immediately as you do not have any professional commitment.