If you haven't done exercise for some time, maybe even years, you should start very gently with exercises designed to gradually increase your stamina.  Going all out is counter-productive, placing too much stress on your unprepared body systems.

Gentle exercises that involve a mix of standing and sitting postures will help ease you towards greater body strength, without being too much. As you progress with these, then you can gradually change to doing a greater variety of standing exercises, designed to loosen you up and help you relax mentally too.

The key is persistence - do a few exercises several times a day and this will help you achieve a manageable change that you can see and measure. If you miss a day for whatever reason, don't worry about it, just go back to your gentle exercise regime the next day. Keep at it and eventually you will feel so much happier, as you are able to get around and manage more and more.

I took up tai chi after years of being a couch potato, so I speak from experience. Nowadays, I feel very much more incontrol of my life and happiness, as well as feeling 10 years younger!