I've read to gain strength you should lift heavy weights with less reps., for endurance (lean/cut) you should lift lighter weights with higher reps.

Right now I can do bicep curls with a 30 lb. dumbell - 3 sets of 8 reps., Should I lower to a 20 lb. dumbell - 3 sets - 15 reps.?

Post your routine up and we can help you out - what are your goals right now? To be more lean, to bulk up, to gain strength, endurance, etc.? It'd be easier for us to determine

Well, I don't really have a routine right now, first trying to figure out which routine would work best for me. I weigh 175 lbs., 5'11" 26% body fat, 26 yrs. old. Typical calorie intake is less than 2000, but I'm working on that to keep it atleast at 2000. I have a beer belly which I'm trying to get flat. Also gain some upper body strength but nothing too much, mostly I want to get lean and cut, and maybe get into some boxing after I get into shape. I think before I try to get cut, I have to gain some strength to have some muscle to cut (does that make sense?) If I can only lift a 30 lb. dumbell for 3 sets of 8 reps., that means I need to gain some strength first since that's really weak even if I'm not looking to bulk up correct?

doing 3 times 8 is better than endurance, why do you want to build endurance, you should be adding more muscle instead as this is the principle fat burner.

stregnth gains is really about only 1-4 reps, doing 8-12 as you are doing will incurr a little stregnth but primararly hypertrophy.

So are you saying I should max my weight to 1-4 reps.? how many sets? I would actually prefer this since I start to get lazy doing that many reps. I dont' really care about endurance, I just want a lean/cut look. I just don't want to get bulk where it makes my head look like a grape compared to my body.

edit: what exactly is hypertrophy? I searched online and couldn't find an easy to understand explanation.

No. Your best bet is to do both higher and lower reps. For example. If you train 4 days a week do lower 2 days and higher 2 days. Another way to do it would be to do lower reps in the beginning of the workout and higher reps later on.

The point is that you will have better results training in all different rep ranges.

Strength increases come not from a specific rep range, but having the work capacity to train in many rep ranges.

If you want to lose body fat.Start doing cardio. At least 3 days per week. Your general condition will get better due to this as well as increasing fat burning. Strength goes up more if you are in better shape.

Stick with the 30 lb. dumbell - 3 sets of 8 reps and work your way up to 15 reps and then add weight.
Lay off the beer. Beer lowers your testosterone( you need testosterone to build muscle).
You should do a fullbody workout with compound exercises. Why not do the boxing right now? The boxing will help get you lean.

I don't have a punching bag at home, and don't have a gym membership at this time.