Big question, easy answer; or should I say ANSWERS? There are a ton of things you can do at home to easily build muscle in your spare time, and I'll just mention a few of them today to start you off.
Start by getting up earlier and waking your body up, ready to face the day.
Stretch a little and allow your muscles to take things in.
If you take vitamins - a great idea for building muscles - now is the time.
If you're anything like me, it's tough to wake up in the morning, so go for a walk to start.
Upon your return - provided you're feeling awake and your muscles feel relaxed - start by doing just a few sets of easy push-ups (search online for a quick guide and/or video to make sure you're doing them properly), then get back to your daily routine.
After lunch time, take a few minutes (after another short walk) and do three to four sets of crunches.
Again, if you're not sure how to properly do them, I recommend finding a free instructional video online to prevent injury to your back.
Another great way to build muscles at home - and this is a weird one, I suppose - is to flex the muscles in your buttocks while you're watching a movie.
Do this every night for a few months, and I'm telling you: you will notice a major difference! Turn the flab into nicely toned muscle, and start getting those looks from the opposite sex that we seem to all crave.
Happy building!