Easiest way to get a lan running with internet is the following.
1 router which acts for several things:
- delivers ip adresses in the group so everyone just plug in and can go do stuff.
- bandwidth limiting of the internet per ip. This so people dumb enough to download torrents on a LAN which happen more often than not don't kill the internet.
- can setup QoS so gaming and IM can go before other stuff.
beyond that you use switches. For 30 people I would say 2 of 16 and 1 of 8 ports. Why you might ask well on the 8 you can put servers and on the two of 16 the pc's. 1 of 48 might do the trick but you need some more cabling.
beyond that make sure not all pc's run of the same fuse. In Europe a group can handle 16A which is about 3500 watt basically meaning 6-8 PC's before things start getting bad ;) How it's in the US I don't know exactly but you need to calculate around 500 watt per PC when it's gaming.
On a lan you DO NOT disable firewall, anti virus programs and other stuff. Your PC might be safe but others probably won't.
You do want to turn off your public file and folder sharing. Nowadays it's less of an issue than in the Windows XP days but still having open folders for everyone to add stuff on your pc is not a good suggestion.
as for snacks.... people generally bring their own stuff to a lan just make sure you have some options to get some meals or make some stuff by yourself. With drinks people generally use the regular stuff like cola, beer, juices, energy drinks, tea and coffee.
As for games it's no way to tell. just ask among the people who come. We generally do some Warcraft 3, Borderlands, Trackmania Nations and Serious Sam for the lolz beyond Battlefield, League of Legends and god knows what is out around at that time.
One real big advice would be to have someone who knows a few things on networks and router configuration. It doesn't have to be pro's but someone who knows how to deal with issues that might arise and basic configuration and you can have a blast.