You can read 100s of threads on this but my best advice would be to give it a go and your rhythm and balance will come in time.
Start off holding the front of the saddle and squeeze with your legs to ask your horse tp trot. Start off in sitting trot until you feel the need to rise, you should get the need to rise (post) as sitting trot is uncomfortable! You should sit when your horse's outside leg (closest to wall) comes back and rise when it is forward.
With practice, it'll be the most natural thing in the world!