I did a whole bunch of reading because I love guinea pigs and seriously considered getting a pair awhile back.
Guinea pigs are very social animals, so please seriously consider getting a second one to keep your guinea pig company.
Also, pet store cages are too small for happy living - the information on
http://www.guineapigcages.com/ is very helpful, and shows how you can custom build your own interesting, fun habitat fairly cheap. There's also lots more information including health, feeding, exercise, etc.
Guinea pigs need some floor time each day - time out of their cages to play on the floor. Somewhere safe where they can't get trapped under/behind furniture. Consider a large bathroom, or blocking off the kitchen, with some fun, safe toys (note that guinea pigs can get stuck in toilet paper rolls).
Just found this page -
http://www.guineapigcages.com/toys.htm
Top priorities:
1) Another guinea pig
2) A larger cage
3) Floor time
4) Hidey house
5) Fresh hay
Try laying a paper bag on its side for a hidey-hole that's fun to chew!