You need to get a pair of clippers, and have the blade sharpen before using them, don't waste your money on a cheap pair.
I have always bought Oster or Cattle Master clippers
But before you invest in a pair, see if you can barrow a pair to see if your horse will even stand for clipping.
If your going to be showing this horse clipp afew days before the show, or if it is just for grooming,then do it when it starts to get a little stubble around the nose, bridle path depends on how fast the mane grows, ears I guess that all depends on how the horse is kept, stall or pasture.
I only clipped my show horses ears, the ear hair helps keep the dirt and bugs out.
The only thing I clip on a regular basis is bridle paths. Mine all have long manes and it just makes life a little easier.
Show clipping is a bit more in depth. I do legs about a week out fairly short, less work right before. Ears, muzzle, cleaning up jaws/throatlatch, eyes I do the day before.
I have used Oster Golden A5's for the last 20 years and love them. You can pick up the older styles relatively cheap used and then put the money into new blades. I keep several sets of 10/15/30/45 blades. Keeping blades clean & oiled after every use will help extend the life between sharpenings. Kool Lube to spray during clipping, keeps blades from getting warm and it smells really good ;)