My band is about to play its first gig at a club, we are renting a 500w PA with two 15 in speakers and a powered mixer/amp. We are going to mic drummers bass drum and have a singer and back up singer. I was wondering if you guys had any tips with working a PA, the mixing board is pretty simple, and I was also wondering if that should be loud enough.

500 should be good enough....but I'm no expert on live gigs...
You should be most worried about having enough inputs for all the mics/instruments and get someone who knows how to get a good mix going for you.
i dont know about the 500w

i have a 500w as well (2x200w) and i can tell you that if we play in our 25m2 room, you won't have a chance to hear the vocals, just because the drums are too loud.
yea, make sure you get someone experienced to do the mix. it's pretty tough.
Well we practiced with it the other day and we could hear the vocals just fine and we had it half way up.

We arent micing anything except for vocals and bass drum.

Im more worried to see if we need a monitor or not.

Also I noticed some bands put the PA in the back as opposed to the front, is there any advantages or disadvantages to that?
well, the advantage if you put it in the back is, that you might not need any monitors. but i'm afraid you will be stuggling with th efeedback from the mic if you put it in the back.