Be aware of any physical limitations/injuries you have before you begin an exercise program and modify your movements accordingly. If you are unsure what these injuries are or how to modify your exercises then see a doctor/physical therapist and/or hire a personal trainer. Hiring a personal trainer even for a short period is an invaluable investment. I know it is in my interest to say this, but it is really the case.

What a good personal trainer can give you is injury prevention and knowledge on how to work out. Not everyone can afford to have a trainer workout with them at all times, but at least you should have some guidance in the beginning. This investment will pay off in time saved not knowing what you are doing at the gym. 

As you start resistance training, it is vital to allow your ligaments time to adapt to this new working reality. They are much more likely to be injured during the initial period than your muscles are. In order to do this you should build up the load (i.e. weight/difficulty) gradually. Also you may have to tailor the time of day of your workout to sync with any side effects you may be experiencing from medication (nausea, lethargy etc). Mornings may not necessarily be the best time for you to work out. Experiment with this. 

How should you organize your weekly workouts? A favored method is breaking the week up into body parts—chest day, back day, leg day, etc. This is an old-school bodybuilding way of looking at the world which has the advantage of taking the thinking and guesswork out of what to do but ultimately gives slow results and becomes very boring very quickly. We are concerned with adding on some muscle mass but not in bodybuilding, i.e., the sport.

I suggest favoring compound movements, which are movements which use more than one muscle group. Squats, presses, deadlifts and jumps are all in this category. These work your body in a highly functional way and can yield great results in terms of muscle mass addition and mobility. They also mimic movements in everyday life and as such have a practical application (lifting groceries, moving furniture, etc.). Isolation exercises—bicep curls, lateral raises—do not provide any of these benefits. If you are not familiar with these movements and are unable to hire a trainer then consider YouTube as an excellent resource for videos on how to execute them. 

For many people, the gym is an unwelcoming environment and one which does not motivate them to succeed. It is by no means necessary to go to a gym to begin your work out regime. You can carry out an entirely effective resistance training program at home or outdoors, using your body weight. Squats, push-ups, planks, sit-ups are all excellent exercises which are compound and highly effective. Check out thetravelingwod.com for daily workouts that do not require any equipment and which are highly effective. Video links are provided for the exercises. 

Whether you choose to join a gym, workout at home or take up a sport, the most important thing is that it should be something that engages and excites you. So often I see people begin a gym workout life, very enthusiastic and motivated, but after a few weeks or months they drop off, no longer drawn to the activity. This is because they have not found a passion for it. So often we are presented with the gym as the only possibility for fitness but this is so far from the case. Look into sports or group activities. Explore Crossfit, which is an outstanding, motivating way of achieving peak fitness and which also provides an excellent social and encouraging environment. See crossfit.com. Your exercise regime should be joyous and fulfilling, and if it is not, it will only become a source of irritation and ultimately something which you will not stick to.