All very good points on here. I can't emphasize enough the importance of what they've all said.
However, I would just like to add that it will probably take awhile before you start seeing improvements if you're just starting out. At least, I didn't see any for what seemed like a long time.
The first thing I really noticed (finally!)when I started was that my waist size began to shrink--everything else appeared the same (even my cardio times, and weight amounts never improved even though I was supplementing, dieting, resting, and lifting properly), but I was suddenly notching my belt a rung tighter than I ever had before. And then it was not long after that point that I started to make "real" progress in my appearance, bf%, strength gains, endurance, attitude, and other indications of health.
I guess what I'm saying is, don't get discouraged because you're not seeing or necessarily feeling any changes at first--it took me over month before I noticed my waist had shrunk a bit, but once that happened, it was like a viscious cycle had started and soon after I was seeing improvements all over the board. Now I look forward to kicking my own butt on a regular basis. But that first stretch was a real bitch to stay motivated through, especially when I kept feeling worse during the days after my workouts.
Trust me, if you can keep up with it, there'll be that one time, perhaps when you're about to jump in the shower after your umpteenth killer workout, when you'll catch yourself in the mirror and say, "Holy shit! Where the hell did those come from!?"--and suddenly all the punishment just became worth it.
At least, that's what I think I'm finding out right now. I've still got a long ways to go before I reach all of my goals, but there are many unexpected rewards along the way that keep making it easier and easier to stick with the plan. My problem now is that I've got to watch that I don't overtrain and injure myself, which for me was unthinkable only two months ago.