As Wearia said, start slow. When I first decided to start a work-out regiment, I did several sets of several different exercises and, although I felt fine for the rest of that day, I woke up the next day in agony. It took almost a week for my muscles to heal enough to continue the work-outs. So, even though you know you can do, say, three sets, do only two and work your way up to three, and then four etc.
Start with eight reps of any given exercise and work your way up to 12. When you can do 12 reps of a certain weight without difficulty, increase the weight and start over at the eight rep mark. Continue to repeat this pattern and watch those muscles grow!
Limit your sessions to no more than an hour, especially if you are strength training.
I also recommend supplementation, but that is, of course, up to you. I currently take about five grams of Creatine Monohydrate and 20 grams of whey protein a day. Don’t be fooled into paying those exuberant prices for stuff like Nitro-Tech or Cell-tech just because they played around and added some Lipoic Acid or free form glutamate. The cheap stuff works just as well. You will see and feel the results of supplementation.
Most importantly — Discipline! Stick with your routine no matter what. Don’t tell yourself that you’re too tired today and you’ll make up for it tomorrow. Don’t make excuses! Your body will adapt to the absence of your work-out just as it will to the presence of it. The more sessions you blow off, the more out of shape you will become, and the harder it will be to get back to where you were.
Don’t be self-conscience about looking at yourself in the mirror. Do it. Do some poses and flexes. The longer you maintain your routine, the greater you’ll look in that mirror. Of course, if you’re starting this to feel better and not to look better, then don’t bother with the mirror.
As joemama24_98 said, not everyone can look like the bow-flex guy. He is just one guy, the best looking guy, out of probably hundreds who answered the casting call. Don’t expect to look like Schwarzenegger any time soon, if at all. Again, if looking great is not your objective here, then disregard this piece.
Most of all, enjoy yourself and know that you’re doing your body good!