Start lifting. Get a small set of home weights. Test them at the store and find a weight that's easy for you to lift several times, one that pushes your limits slightly, but you can still lift, and one that you really have to work at.
Start with the lightest one the first few weeks. Do a couple sets with about 10 reps each. This is just to warm your muscles up, get them used to the idea of a good lifting session. After that, move on to the next two weights. If you're using the middle weight, use the same routine as before. If you're using the heaviest, do a couple sets, but keep your reps low- 4 or 5 will do.
Eat a high protein diet to help build you muscle, but keep away from unhealthy foods you may normally eat. Treat yourself every once in a while, but don't eat them all the time.
Work your lower body as well as your arms. Work your core well, too (back and abdomen). You can't get any stronger or any bigger by working just your arms, or just your legs, or just your core. You have to work all of them. You're body's a machine, you need to keep every part well oiled and working or it'll start failing. You'll plateau and that's never, ever good.