I have 2 guineas, very awesome little guys.
Main things is a nice large cage as they need to run about a lot which leads to epic popcorning, and lots of vitamin C rich snacks i.e. broccoli and spinach are what my 2 love with the occasional little piece of apple and some carrot to nibble on. Just remember they always need hay and they will get through it like no tomorrow!
I'm looking at expanding as we're moving them into a larger area in the back garden, so will have plenty room and grass to graze!
They're amazing escape artists as well... mine are like ninjas!