The trot is a two-time pace in which the legs move in diagonally opposite pairs. The sequence is right front and left rear followed by left front and right rear.
The trot can be ridden sitting or rising.
When in rising trot your bottom should return to the saddle on the right diagonal for a left circle and the left diagonal for a right circle. This helps to distribute your weight more comfortably for the horse. Your bottom will be in the saddle when the inside hind leg is on the ground and out of the saddle, and so freeing the inside shoulder, when the inside foreleg is on the ground.
To check you're on the correct diagonal glance down to the outside shoulder and you'll see it move back towards you as you sit in the saddle.
To change from one diagonal to another, which you will need to do after a change of rein, sit for an extra beat (bump) in the saddle and then resume rising again.
When out hacking, you should also be aware of and change diagonals on a regular basis. Often a horse will have a preferred side, and will 'throw' his rider onto this diagonal more often than not. If the rider allows this to continue, the horse will develop unevenly, with stronger muscles on one side than on the other. This will eventually result in a stiff horse who is less able to bend in one direction, and will therefore be less capable of more advanced work. Ensuring both diagonals are equally used during recreational riding will help to avoid this.
Next - the canter.