Well, grooming before riding? Heck, sounds easy enough!
Do you mean grooming the horses before they go into lessons, or grooming at a show?
If you are just cleaning the horses up before a lesson, make sure you clean thoroughly under the girth area and saddle area to make sure there is nothing that would poke the horse once the saddle is on. Pick all 4 feet too!! =)
And remember, when you are working for someone, what they say goes.. and when working with a particular horse, whatever the OWNER says goes.. if you are grooming a horse and some kid walks up to you and says you need to stop... well.. tough nuggets to them.. but if the owner says you need to stop... Stop! LOL Be nice, try not to complain or seem frustrated, and don't be upset if they sorta boss you around and treat you like you know nothing. If you arent sure about something, Ask!! =) You will start building up a positive rep. In no time!
Good luck!! And have fun!!!