As said, make sure that the horse is completely clean and dry. A shady area is great for both of you. You will need good, strong clippers. Dog clippers won't work. Oster A-5, Double K's something like that. You don't have to do the entire horse if it's first time for you and the horse. I like to start with the legs, when the horse is calm and cool and hopefully you are too! Don't worry about a few clipper marks, they will grow out! But you may be more comfortable doing the body area.
Keep checking your blades to make sure they aren't hot. A quick tap on your arm or leg will tell you, or the horse starting to twitch and dance will let you know also. If they feel hot, it's time to change them. You will need several sets of blades. For just taking winter hair off, I use size 10. I usually use 3-6 size ten blades for the first spring clip per horse.
When you are showing, you will need to clip different areas with various sized blades. Laura of Ten-L training has a great step by step for clipping, especially the head area.
Clipping is fun, but a lot of work! Some ice water, shade, and a fan are nice when it's really hot. Be careful that you don't put a freshly clipped horse out in direct sun for hours - they can get sunburn just as easily as humans.