I've donated twice. The first time I was in school and donated to Locks of Love - the school had organized a free cut and took care of donations. Second time I donated to Pantene Beautiful Lengths since they accept shorter hair. I went to my normal salon and they cut off my ponytail and then got a regular cut. I mailed in the hair myself.
It's not a complicated or expensive process, but be sure to review the requirements before chopping off your hair. A lot of places won't accept gray, colored, or treated hair, and there are varying length requirements. As Iris hinted, a lot of charities sell off unusable hair to offset the cost of making wigs which may be objectionable to you. If you're looking to make it free for you, you might look around at schools or other charitable organizations that have upcoming donation events.