I have been lifting wights since I was 15 and I will tell you this, if you are going for definition, then you should shoot for more reps at 70% of you max. That way you are challenging your muscles to go to "unchartered territory". If you are going for more mass ( which I don't think you want to do given I know your size) then you should go for pure power. What I did when I played football in high school is that I started at doing an X number of reps at 70% of my max and did less reps till I hit my max ( I had a spotter mind you).
If I was going for definition which I presume you want to do ( I could be wrong) I did what they called "burnouts". I started off at my max and worked my way down till I hit about 60% max. What I did was I took 10 pounds off and steped UP the number of reps.
Overall, if you want to change the appearance of your body a good way to go would be to incorporate the weights with a good regimine of cardio. As you saw in my thread of how I lost 55 pounds, that is exactly what I did, It paid dividends, mind you.
If you ever want to talk further about it. I'll be there for you. If you want to keep it private, feel free to PM me.
It's good to see that you are taking steps to improve your health and if I can I hope I can make a difference.