I have been lifting for several months and am reasonably strong. I started with P90x a year ago, and since have continued on my own- 2-3 days/week. I do about 10-12 reps depending on the exercises (like shoulder presses, lat pul downs, chest press etc), except for things like push ups (can do 30-35) and pull ups (can do 1-2).
Is this lifting heavy?? I keep reading that we all should lift heavy things, and I don't know what this means.

How many reps should I be doing? Do I need to increase my weights so I reach failure at 2 reps? 5 reps? Also, are there certain exercises that are optimal and others that are not? Anyone have a good female lifting routine they can share?