what to charge for horse photography? Also pictures!

I have a few questions.... I am a beginner photographer that would like to start taking pictures for people I don't know( I have done it for friends) - Anyway, what does one charge for a picture? Do you charge by the hour? Or the picture? What do you charge? Also, gas mileage? If your a photographer, please comment and let me know thanks

NOTE: here are a few pics that I have taken... what do you think?