When you get on your horse and you are situated, squeeze your knees in towards the saddle. When you do this your horse will start to walk. Do not squeeze your knees for a long period of time because that will cause your horse to start trotting.
After you are done exercising and walking your horse for awhile you can begin to start trotting on your horse. When trotting you follow the same steps that you use for walking. So position yourself with your shoulders square, your back strait, your heels down and toes pointed in. When you have done all of these basic steps, there is a new step to add in for your horse to start trotting. This new step is to make a clicking noise with your tounge on the roof of your mouth and this should get your horse to hopefully start trotting if he/she doesn't then you did something wrong.
For cantering you follow the same steps as walking and trotting so, squeeze knees, sit square, have high confidence, keep your back straight, and make sure that your heels are down and your toes are pointed in and up towards your horse's armpits, also instead of the clicking noise with your mouth you would make a kissing noise like you would do to dog to make it come to you.