Bulking up
the fact is, muscle requires lots of food to be built up. The art of bulking up is very challenging. it takes a lot of patience and time. The fact that some people feel they ARE bulking up is actual more like their fattening up (sorry for the lack of politically correctness there). They are eating unhealthily simple as that.
some of the pumped up feeling during exercise is simply fluid coming into the muscles. that dissipates after an hour or so.

What weights to use. General conditioning is typically any where from 10-20 reps of an exercise. that being said, if all you can do are 5 pushups, then thats what you start with. you want good form from beginning to end.

Muscle Failure
you want a nice challenge to the muscle, but you don't have to take it until you can't lift any more. you want to take it until you can't lift any more PROPERLY. there's a subtle difference. you will usually know because you will start to compensate (like arching your back to throw the back in, or swinging the body...stuff like that).

Remember the purpose of training is to apply the stress to the body that the body adapts to. Then you apply a new stress (ie heavier weight, more reps etc). thats progressive training. in order for this to be effective, exercises must be structured, other wise your body has nothing to adapt to. you have to remember to change your routine every couple of months.

hope that helps

hannah