Warm up before workout, but dont make your muscles TOO limber. Just enough to get your body moving and whatnot. I wouldnt take 15 minutes to stretch every muscle. The main idea is just warming the guns up.
To build bulk, do heavy weight and a few reps. 3 sets or 3 or 5 sets of 5 or a 5-4-3-2-1. Pretty much anything along the lines of low reps and high weight. This will increase the bulk of your muscle. If you wanna look like an NFL linebacker, this is how ya start.
To get toned, do less weight and a lot of reps. like you said you were doing, 12-15 reps is decent. The typical sets of 10 that most people do is just sort of a median of working out. It will gradually build bulk and tone, while if you focus on low or high reps, you will increase tone faster/slower, and same for bulk as well, depending on what you focus on (if you to lots of reps at low weight, your not going to get much bigger, but you will get toned, and vise versa)
After weightlifting, its important to keep loose while your muscles start to heal themselves. I ussually try to keep limber after lifting, stretch before i go to bed, stretch in the morning, ect ect.
Protein intake is also important. Make sure your not just taking in sugar and carbs. Protein/amino acids take a big role in weight training. And if your looking at an athletic point of view as well, other things will take effect such as how explosive you lift, and if you are lifting correctly. Go through the full motion of each lift. When your pumping the guns and doing some curls, extend your arm(s) all the way straight before you curl back up, when your benching let the bar touch your chest (wouldnt bounce it off the chest if i was you hehe), ect ect.
if your working one muscle, lets say your biceps, make sure to work the opposing muscle equally as well (which would be your triceps). not doing so will result in a bad ratio, causing your weaker muscle to more easily cramp/tear/damage itself.
And what i like to tell my buddies when lifting is dont be afraid to get pissed off. Adrenaline is a HUGE part in weightlifting. You have all heard the random stories of some 110 year old (exxageration) lady picking a car up off of somebody. Once you get that adrenaline pumping, your going to be able to lift more then what your muscles are used to, resulting in improvement. Ya gotta want to do it.
Ah so much more i could say, but i dont have the time atm lol. If ya need any info PM me.