Ah-ha, you hit it! Motivation is key. You're absolutely right, you gotta want it bad enough. Took me a month of groaning and b!tchin' to keep going to the gym. Glad to hear you're not overweight or anything. That's going to make it that much easier on you.
I think I also may have forgotten to mention: be sure to get a complete physical before starting anything. I know, it sounds like your momma talking, but sometimes it's possible there's something going on you don't know about. Better safe than sorry.
Don't worry about having enough energy at the beginning of a workout. Just do it at a sensible time. If you get tired, good! Eventually, you'll break out of that. Just like when I work out, it feels like my arms are going to fall off for about a half hour....then the remarkable feeling of those muscles pulling kicks in.
I used the term "beating the crap outta myself" for a while.
Oh, BTW, I'm 22, diabetic, asthmatic, an ex-smoker, and allergic to (almost) everything natural, not to mention a college student.
Every condition I have has improved greatly in manageability since starting to work out.