How to Ollie (besides the obvious)(gr or concrete)?

Im a new skater and need some tips on how to do an ollie. Anythng besides how to specifcly do it, or to try hard or anyhtng like that. One thing is should i do it on gr or on concrete? The gr seems like itll hurt me less, but it also seems like the back wheels wnt move. Do they have to move, even a bit, for me to nail the ollie?