Everything you need to know at the beginning is written up in the
"BHS Manual of Equitation" ISBN 0-872119-33 6 - you can buy it through AMazon
. I suggest you buy it.
The instructor should be one who claims to be able to convert a capable Western Rider to the ENglish system - you don't need to be taught to ride - you need to be taught to ride English. He/she should come recommended.
It would be best if you learned to ride on a horse which has been trained to ride English on a saddle which fitted the horse and hopefully which fitted you. The correct fitting of the saddle is important. If you are a big girl then you might need a bigger saddle.
However big girls do ride English but they usually ride big horses.
The key differences are - a slight change of sitting style best found whilst sitting on the saddle. You sit upright no slouching. Toes up, heels down on the three seat bones.
You hold the reins in two hands and get contact with the horse's mouth
Through the bit - best type would be a simple snaffle. You have to learn to feel the horse's mouth at all paces. You ride "in contact" - no neck reining
The trot. You must learn to post to the trot. Not difficult but a knack.
(Sitting trot comes much later)
There is no horn to grab - it is easier to fall off a tiny English cut saddle but you can ride already so that should not be a problem.
You should not do any jumping until you have mastered the trot.
It will take some time to perfect the method but once you have learned it then you should experience a better feel for the horse because there is less between you and the horse's back.
Any horse can be ridden "English" but it would be nice for you to find a horse which knows what you are trying to do. You have to learn a new set of cues (aids) but if a kindly horse knows them already then it will forgive you for not knowing. The instructor just stands in the middle of the arena and shouts - a good schoolmaster horse does all the teaching.
I am not sure why you guys want to learn to ride English but all I can say is that the effort is worthwhile. It is important however, at the beginning to learn to sit correctly in the saddle. If your seat is not correct from the word go, then you will develop bad habits which will be difficult to remove later on.