The kind of food you get depends on what kind of party you're throwing. For all all-campus event, you want mostly snack food, and you want it to be cheap. It's important to get enough beverages, since Swat parties are infamous for running out of non-alcoholic drinks very early.
If you want to have a party with alcohol, you have to do a bunch of fudging receipts for SAC and stuff like that. However, if you're reading this and you want to have an all-campus party with alcohol, then something very odd is going on. At any rate, you will need to get a party permit, which you can get by asking the student activities coordinator. It will be a sign that says it's illegal to drink in the Commonwealth of Pennsylvania if you're under 21, and you'll have to put it up somewhere.
Swat currently has a set of people called Party Associates who go to all-campus parties and look after things. SAC encourages you to have a couple of those, although it's not required (i don't think) if you don't have alcohol. Using Party Associates has the advantage that if you want to enforce a no-drinking policy, you can get them to do, so you don't have to pick fights with people at the actual party. A warning: PAs currently check everyone coming in the door for a Swat id. Make sure you tell them in advance that alumni will be coming to your party. We forgot, and had some trouble with this.