Your relationship with the tutor must be sympatico - and the tutor must know what tutoring is all about. Not all certified instructors are good at teaching.
The horse on which you are learning is of signficant importance - and I would always say that it is the horse that teaches you to sit - the instructor meely tells you how. You'll need a schoolmaster horse that knows its business
At the beginning one to one must be the best - even if it means that your lesson is cut down to half an hour.
Your position in the saddle is of prime importance at the beginning. You must learn how to sit correctly and you must maintain that position throughout the ride. Remember toes up, heels down, feet straight. And sit up straight in the saddle - head high. Then relax. All must come automatically - without conscious effort or tension. Don't leave the gap between lessons too long.
Then when the tutor reckons you are on message, try arranging lessons on the horse you plan to ride.
And also go for some pilates exercises. You'll need for your centre core muscles to be tuned up, your calves and hamstrings to be stretched.
Get a friend to take some videos and sit and study them when you get home.
And above all enjoy yourself. It is worth it.