I don't know what your budget is for this party. We have parties fairly often with DJ and etc for our daughter (13) and her friends.
A lot of hotels and public golf courses will rent out an event room for you. Sometimes there is a minimum food purchase which is easy to meet if you have several guests. We attend a private golf club and that is the route we always take.
If you go the hotel/golf course route, the hotel or course can provide the appetizers, drink, etc. You can decorate and provide music. A GOOD DJ will run you around $1500. As for food, the biggest hit was mini pizzas, veggie and fruit trays, chocolate fountain with things to dip and make your own sundae.
Now, this past weekend our daughter had about 25 teens to our home for a halloween party. We thought about a DJ but we ended up buying a Bose speaker from Best Buy (around $300) that she can dock her Ipod or Iphone on. That worked GREAT and the music was perfect for them for the entire party. Several of the other teens here had Ipods and/or Iphones and they took turns with the music they each had downloaded.
The food we used at our home party were the veggie and fruit trays, lots of sliced apples with caramel dip, and pizzas.
At first we were skeptical about the Bose but the customer service rep at Best Buy told us without a doubt it would be a hit and we consider that we already got our money's worth out of it. It will be used a lot around here!
Have fun with 18! This ia a huge one. I am busy planning the 14th in December!!