Dependant on your airline and destination - you may not need to buy food.
So where are you going and on which carrier? Also how long is your flight?
First time flying - hmmm. Once through security you go to your departure gate when it is announced (its not really announced you generally have to watch the boards) Then you wait there until they call your row/seat number which is on your boarding pass. On some airlines its rush and grab - that is not a fun time.
In your seat you buckle your seat belt and wait. There will be a safety demonstration to which you should pay careful attention as a first-time flyer. (you should also pay close attention if its your 1000th flight but many people do not:) The plane gets pushed back slowly from the gate and then it trundles for a while until it gets to take-off point. Then it goes very fast and lifts-off. Don't be surprised if the plane squeaks and creeks and grumbles - some of them do that. Also don't be surprised if there is a bit of bouncing....then the plane keeps climbing and turning until its headed in the right direction and at the right height and then you can probably take your seatbelt off and relax. A window is nice to see where you're going and there may also be a map if you're lucky enough to have setback TV.
Have fun :)