Hi Denise, I went through the same thing recently with my guinea pig Stella, so I can give you a bit of advice.
My girl was given to me as a complete surprise and was kept in a cage with males before I got her (grrr). Of course she was already pregnant and sooo young, only 3-4 months old. I started researching all I could to prepare for her delivery. A good place to start is Guinea Lynx: http://www.guinealynx.info/breeding.html
Fortunately most guinea pig deliveries go well and the mom knows what to do right from the start. However make sure you have a vet lined up in case of emergency and also read up on signs of birthing complications. Go ahead and baby-proof your cage (I used 1/4" hardware cloth on my C&C cage). Her delivery could happen any day and you don't want the babies escaping if you're not around. Lower the water bottles so the pups can reach them and separate mom from all other cagemates. She should have her own little private hiding spot to give birth in.
My Stella had her litter early in the morning before I was awake, so I found her with four newborn pups and all seemed healthy and alert. I weighed them all right at birth and every day from then on until weaning (3 weeks). On the second day, I noticed they dropped a few grams, which is normal. Then they started gaining steadily and I kept an eye on them to make sure all four were nursing. Go ahead and get some Oxbow Critical Care on hand and a syringe in case you need to handfeed a weaker pup. Also keep an eye on mom to make sure she is eating and drinking normally.
Most first-time mothers do fine, even if they are young. Stillbirths can happen so you need to be aware of that. Everything should go well, just be prepared for the worst and then maybe it won't happen - that's my theory in life! Good luck and feel free to PM me if you have any specific questions.