What has to be in the house first, what can wait, and what not to buy at all.
Before you spend anything
Guinea pigs are cheap to buy and not cheap to keep properly. The two things people get wrong are keeping one alone, and forgetting they cannot make their own vitamin C.
Before they arrive
Two or more, same sex or neuteredGuinea pigs are herd animals. Switzerland made keeping one alone illegal. A single guinea pig is a sad guinea pig.
A cage far bigger than the pet shop suggests7.5 square feet for a pair is the usual minimum, and more is better. C&C grids or a large plastic base beat anything sold as a starter cage.
Unlimited grass haySame as rabbits. It is the diet, not the bedding.
A daily source of vitamin CThey cannot make their own. Fresh bell pepper is the easiest — half a small pepper between two, daily. Scurvy is a real thing in pet guinea pigs.
Two hidesOne each, minimum. Ideally with two exits so nobody gets cornered.
Paper or fleece beddingNot pine or cedar shavings — the oils irritate their airways.
The first 48 hours
Let them hideGuinea pigs freeze when frightened. They may not eat while you are watching, so leave the room and come back.
Check both are eating hayWithin a few hours.
Talk near themThey learn voices fast, and it settles them faster than handling does.
The first week
Handle them daily, brieflySupport the back end. Start with lifting them onto your lap for two minutes.
Introduce vegetables slowlyOne at a time. Sudden changes cause diarrhea, and diarrhea in a guinea pig is serious.
Find a vet who sees exoticsSame warning as rabbits.
Weigh them weeklySame reason. Weight tells you first.
Can wait a month
A run for floor timeOnce they are settled and confident.
A second level or rampGuinea pigs are not natural climbers — get the floor space right first.
Nail clippersYou will need them within a month, but not on day one.