How Much Weight Should You Lift
The answer to this question will depend on the reason you are training and what your specific objective is. But generally to increase both the size and strength of a specific muscle group you should use a weight that is around 60% to 70% of your maximum or 1RM (one rep maximum).
Increasing strength should be about 1 to 8 reps per set, increasing muscle size should be between 5 to 12 reps per set. Muscle endurance should be trained with 12 to 20 reps per set. Probably the best overall combination for muscle endurance and muscle size is 8 to 15 reps.
If you know what your own specific objective is and why you are training with weights then it should be easy to work out the ideal weight that you should be focusing on. The strength curve will be different for each individual so you need to work out what is best you.
Form is everything including warming up correctly doing reps with no or very little weight before you start to train heavy. Working out your own specific strength curve takes time and practice so if you have been training less than a year then keep a training journal to plot your strength curve.
It is a good idea to use a slightly heavier weight when warming up as you get stronger but never disregard warm-ups, without warming up you will cause an injury. A bodybuilder whose bench-press is around 225 pounds should warm up with 15 or 20 reps with 130 pounds.
It is a good idea to do the difficult movements first in your workout. Your energy will start to sag after your first 20 minutes so go heavy in the beginning of your workout. For example go down to 8 or 5 reps with maximum weight in the first 15 minutes and not at the end of your workout.