Get a left handed mouse. Preferably you want a vertical mouse for both your left and right hand.
Evoluent manufactures my favorite.
Get a good chair with lumbar support. Make sure your knees bend at a 90 degree angle when the soles of your feet touch the floor. Your eyes should be about two thirds up the height of the monitors screen area. Your upper arm should be hanging nearly vertically, with a 90 degree bend in the elbow when you grip the mouse.
Now, all the above is perfectly obvious, but lets go on.
Move your keyboard to the side and put the mouse pad straight in front of your belly button! You don't use the keyboard at all, and use the mouse all the time. The most used device should be straight in front of you. (Which is why the keyboard is 'normally' there.) We are built to work
in front of the body, not somewhere out to the side.
Talk to your doctor, then start going to the gym. Get a PT to help you set up a program to get in shape and stay that way. No need to try and become an athlete, but for the rest of your desk riding days you'll be going to the gym a minimum of three times a week.