Ensure you do your homework / research, know the job description and market rate of the job you're applying for and take into account your qualifications and experience as well.

If you quote too low, your potential employer will probably be happy but will also take advantage of your desperation ... or worse still, will consider that you don't have confidence in yourself, how will you convince them to invest in you as their new valuable human resource?

If you quote too high without justifying your potential benefit to the company, you'll lose out to other interviewees. All the best to you!