Guinea pigs (unlike hamsters and some other pets) are sociable creatures, and are usually all the happier for company, although they may ignore their humans more as a result. If you don't have a lot of time to spend with your guinea pig, or are gone for much of the day, your guinea pig may be a lot happier if you get him or her a friend. Same-sex groups, of either sex, usually get along fine if given sufficient room, although from anecdotal evidence females seem to be slightly more reliable in this respect than males. A male and a female are naturally the best company for each other, but unless you want your female to be constantly making little guinea pigs, you will have to neuter one or both of them.
It's a good idea not to try to breed a guinea pig until you have found some responsible people who would like one of the offspring as a pet. Pet stores often treat small animals very irresponsibly, and you don't want to bring guinea pigs into the world that aren't wanted or will be mistreated.
That in mind, there are a few caveats. A female should not be bred until she weighs 500 g, or is 4-5 months old. Also, no older female should ever have a first litter. Somewhere between the ages of 9 and 12 months, if she is childless, her hip bones will fuse such that she can not give birth naturally, and a later pregnancy will require a caesarian section. Therefore, if you plan to breed your female, or if you do not plan to spay her and the situation is such that she may become pregnant later on, you should probably see that she has at least one litter between the ages of 5 and 9 months. If an older female does accidentally become pregnant with a first pregnancy, you and your vet will want to plan on surgery to deliver the babies, otherwise she will likely die giving birth. In addition, do everything you can to avoid such an accident in the first place (for example, have your female spayed even if you think she won't be near a male), since a caesarian section is risky for both mother and babies. For more information, see "Diseases of Domestic Guinea Pigs" by V.C.G. Richardson.
The gestation period (time between conception and giving birth) for guinea pigs is approximately 60-70 days. Guinea pigs do not normally require assistance in giving birth. The young are usually in no danger from either parent, although you may want to remove the male right away, since the female is able to conceive again within the hour after giving birth. Litters can have between 1 and 8 little ones, but typically have two to four. The males of the litter should be separated from the mother and their sisters directly after weaning, since they are sexually mature shortly after. The babies will probably be weaned by the time they are about 3 weeks old.
It is important to handle the babies soon and often, to socialize them to humans. Like other animals that are born precocial, guinea pigs form their social bonds shortly after birth, sometime within a matter of hours, so human contact is critical during this time to ensure that they establish strong bonds to people. Many people are under the impression that handling baby animals too soon will cause the mother to reject them, but this isn't true for guinea pigs. Lots of love and gentle handling and petting from the start will make the babies grow up more friendly, and less afraid of humans.