While we argue that anyone who is reasonably fit can ski with few problems, we are neither qualified doctors nor fitness instructors. If you have any health concerns you should voice them to your doctor.
Any gym can give you instructions on how to get ready for a ski trip. However, we have had 'tips' from gym instructors who have never even put a ski on, and most of the instruction is about getting big strong thigh muscles. This is great for competent skiers who will be using their thighs a lot as they execute parallel turns. As a beginner, you have other problems.
First, you will use some muscles that are rarely called upon. Try this; stand up with your feet shoulder-width apart. Angle your feet slightly so that your toes point in a little. Now lean forwards slightly, bend your knees, and put pressure on the inside edges of your feet, first on one foot, then the other.
This is what you will be doing as you learn to ski. So we suggest you try some exercises for hip flexors, as well as adductors and abductors. It's also important to do some cardio-vascular if you are not used to it - this will help your stamina and help your body cope with a few hours of activity a day.
Something you won't be ready for is poling over flat ground. This tests your shoulders and triceps in your ams - again, if you go to a gym, describe the motion to an instructor and see if there's anything they can suggest to replicate it. There is a movement on Technogym machinery that is excellent for this.