What Shamus said... Figure out something you will enjoy, works for your body, and that you will be able to stick to. And mix things up to keep yourself interested. Find someone to exercise with, if that works for you.
For me, right now it's summer. I run when it isn't too hot, track my mileage and times. I have to pay attention to my feet, a little pain strengthens things, a little too much pain means I have to stop for recovery.
I hit the weights. During summer, I use the muscles a lot anyway, so I only lift about 2x per week. Running also greatly reduces upper body strength and size. During winter, I pump iron about 5x. That frequency requires splitting body parts on different days... chest, shoulders, back, abs, arms, legs. Everybody is different biologically, so what works for me might not work for others. I can't seem to get lean enough for beachworthy 6-pack abs, my body thinks I'm starving and reacts accordingly.
Motivation? Now I can look in the mirror and remind myself that I don't want to be where I was. In the last year and a half I've lost about 30 pounds besides moving weight to muscle. No, I'm not young anymore, and keeping fit isn't "easy", but I'm doing the best with what I have.