>> Start today: do 100 pushups, 100 sit-ups and go run-walk for 20 minutes. Do that everyday, continuing to set goals. Rain or shine. Even when you feel sick, hungry, tired.
Good lord man, I hope OP is a 20-something in fair shape. I started running later in life, and then working out. Dropping from a desk job to the ground to do 100 push ups or a 100 proper sit-ups if you have never done it is going to 100% motivate you to never do that again.
If OP wants to go exercise, and has never done so, go look up couch to 5k or start off with 10 proper push ups and 10 proper sit ups and then wait 10 mins and see if you can do 10 more. Do that three times. Give yourself some time to build that up.
This reply I am replying to has no idea what kind of shape OP is in. Don't go nuts on day one, you will spend a week (or two) recovering and this will demotivate you. (and I am no expert, perhaps call a trainer).