Does it hurt when a boy puts his penis on girls?

Sometimes. It all depends on the situation. It hurts because of how tight girls are when they haven't had sex yet and how there "cherry" pops. For people it varies, you can feel burning or stinging but it's not mind blowing pain. Most people bleed a small amount of blood but not all. Foreplay is good because if y'all are both tense it won't feel good. Lots of lube helps, if she's happy you're happy! Try to take it slow and be with someone you really care about, that helps as well! And remember ALWAYS wear protection b/c you don't want an unplanned pregnancy to ruin your future.
It may or may not, however an average sized penis shouldn't case too many problems. the pleasure from the sexual experience (assuming there is foreplay including oral stimulation of the clitoris and vagina) should far outweigh any pain from the stretching of your vagina.
That depends on the girl. It hurt me a little, and there was a very small amount of blood - but overall there was more pleasure than pain. My best friend says it didn't hurt at all. Having sex for the first time can be a little nerve wracking. Go slow, lots of foreplay (the more aroused you are, the less it will hurt), and relax (being tense doesn't help). Sex is supposed to be fun, so enjoy.