Do NOT start an exercise program without first consulting with your primary doctor. Moreover, do not try lifting weights unless you are properly taught by a personal trainer. Lifting weights is an art, and can be very dangerous if not done properly.
Always start with very light weights. This helps to strengthen and tone your muscles, getting them ready for heavier weights. It also builds coordination and balance, which help you to do the exercises the correct way from the beginning.
Don’t be afraid of machines. Machines are an excellent way to start working out. They are much safer than free weights, but of course you can still very easily hurt yourself if you do not know what you are doing. But they are a great way to familiarize yourself with the different motions for lifting weights, and they will help to strengthen your muscles to prepare them for free weights, if you do decide to move to free weights. When you do make the transition to free weights, again, it is extremely important to start with very light weights and slowly work up. Free weights take a lot of balance and coordination, and each motion should be done slowly and with concentrated effort, not just bouncing up and down (which is very dangerous).
Remember that the best way to pull or tear a muscle is to put a lot of weight and then while doing the exercise, stretch the muscle as far as you can and then “exploding” back to the start position. For example, if you were doing bench press, if you put a lot of weight on the bar, then bringing the bar all the way down to where it is touching your chest (this would stretch your pec muscles to the max) and putting all your force at once trying to suddenly push the bar back up. Very, very dangerous. When using a lot of weight (which you may want to do at some point if you are trying to gain muscle weight), you need to be very careful on how far you go. And never “explode.” The exercise should always be done slowly, with concentrated effort, with smooth movements. And feeling the stretch at the deepest point in the exercise can be very dangerous; please just stretch before or after exercising, not while doing the exercise!
A good rule of thumb is to do 4 sets of each exercises, and do 15, 12, 10, and then 8 repetitions. This helps to both tone and strengthen the muscles in a balanced way. If your primary goal is to gain muscle mass, then it is reasonable to do 12, 10, 8, 6, or even 10, 8, 6, and 6 again. By doing more weight and less repetitions, you tend to build more muscle mass. By doing more repetitions, you gain strength, endurance, and joint stability.