Contraception (implant)

I am thinking of getting the implant, as the guy I am seeing doesn't use comdoms. I don't want to have the pill as I know I would forget to take it. 
I wanted to know from anyone who has had it... Is it painful?? Also if I go to the gp will I be able to get it there and then??
I am 18 so was wondering if I still get it free on the NHS. Many thanks.