Title says all, first thing's first.
1) Find a horse. They are usually found all over the place but I have found lots in large grassy plains.
2)Depending on the size, you might not be able to ride it, so you should bring about a twenty wheat.
3)Right click the wheat at the horse, the horse will stretch its mouth upon given wheat. If it is small, it will grow, if it is medium, it will slightly grow, some are at its peak if you found in at the wild.
4)Find an empty inventory space and right click the horse to ride.
5)Keep right clicking it, it will buck you off, but if it keeps bucking BEFORE you start riding it, it's too small for riding, give it more wheat.
6)When the horse accepts you, you cannot move on the horse without a saddle, press 'E' and equip the saddle on the horse. Armor is not necessary.
7) You are ready to ride, yee-haw c: