Lots of good advice here. I just wanted to add that after a while the exercise you do today will start to pay off tomorrow in stamina, strength, and energy. You will find after a few short weeks that you have more endurance to exercise and a ton more energy. The more you exercise the more energy you will gain.
Like others have said, take baby steps and work towards small goals. Once you reach a goal, set another one, and so on.
When I first started exercising I could barely walk two blocks. My back would hurt and feel like I couldn't hold myself upright. I would be sweating profusely, out of breath, and my heart would be racing. But when I finished I realized I had walked for 10 minutes. So next day I told myself I have the strength to walk ten minutes and if thats all I can do fine. But when 10 minutes was up I found I could walk another five. By the end of the month I was walking for 30 minutes a day with only 1 rest stop. Plus my heartbeat would return to normal much faster and my breathing was normal and not labored. I was really proud of myself. By setting small goals I was able to accomplish what I originally wanted to do. I just did it in small steps.

