First of all you will need to prepare your horse. If you have your own horse in the family horses lot, better tack it. You can trust it more than a horse you might be riding for the first time. It is important for your safety aspect.
Tack up your horse and take it to the ground. If you have a beach near to your place, better choose it for posting trot. This can protect you from any injury in case you fall off the horse, especially if you are posting trot for the first time.
After riding the horse start slowly first. It is important that you first get yourself comfortable in the saddle, and your horse too should feel in the rhythm. Do not rush at once riding on your horse back.
Gradually pick the movement backward and forward of the horse and act accordingly as the horse picks up speed. Do not rush.
To catch on with the rhythm of the horse you can count 1,2; 1,2; 1,2. This will help you pick up the horse movement and trot on the horse movement. You can count one when the horse's right leg move outside and two when it is back at its position.
Also decide when you will stand up or push yourself up in the saddle and when sit. One way is to stand up when the right shoulder of the horse moves forward, when it takes a step forward, and then sit when the shoulder moves backward.
Just let yourself push up in the saddle. Do not stand in full position, as it can pose a risk of falling out of the saddle. Just let yourself pushed up with slight bend forward when the horse shoulder is out in the front and then sit back when it is back at the old position.
Once you perfect your rhythm with the horse beat, you can start position. It is basically switching of positions too; for example, stand-sit-sit-stand. Or else you can choose your way. In the beginning just start with this and then as you feel more confident and comfortable you can change it the way you want.