No matter what shape you are in now you can start resistance training at any time. The benefits of resistance training are many. You can start by working at home if you want to get a feel for your muscles. There is no need to join a gym unless you get bored doing it by yourself at home or you need to progress to bodybuilding exercises that isolate specific muscles. Home exercises can be hard enough if you follow a program. You can start doing simple exercises for large muscle groups like push ups, squats, sit ups, dips, etc.
Start by recording how many pushups you can do and try to progress in every session you do. If you want you can purchase some rubber bands for exercise in your local sports store and begin resistance training before you join a gym. Resistance exercise will make you stronger, increase your self esteem, decrease your body fat, increase your bone density, decrease your blood pressure and it will help decrease your cholesterol levels as well.
Gyms are usually expensive and take a lot of time. Join a gym only if you are looking to sculpt your muscles with bodybuilding exercises or if you enjoy taking group classes. If you do so it is best to invest in a certified personal trainer for a few sessions so you get an idea of how to work out correctly and not get hurt. Exercise is good for you no matter what kind of shape you are in. Ask your doctor if you are in doubt and get medical clearance before you do any strenous activities.