Definitely get more than one. Sows (females) are so much easier to pair up and bond than boars (males) so if you do choose to get males, make sure you have a very large cage setup. Guinea pigs housed alone can get depressed and develop both health and behavioral problems.
They need a lot of floor space as they love to run around, so make sure you get atleast a 120cm long cage for 2 guinea pigs. If you get 3, make sure you have a 140cm cage.
HAY! Is a must in their diet. It should be available to them 24/7 and they will go through a lot of it :P
They tend to pee and poop wherever they want, I don't think its possible to litter train them. They are messy animals! Though they don't really smell.
They need vitamin C in their diet. A good guinea pig food should have this in, you can also give them vitamin C supplements which is put in their water. Carrot and other vegetables are also good sources of vitamin C.
They are naturally skittish animals so don't be upset if you find that they 'don't like you'. Two of mine are over 2 years old now and they still run away from me. Yet my 4 year old piggie actually runs up to me when I open the cage door and doesn't run away if I go to pick her up.
Guinea pigs are prone to ringworm and respiratory diseases, so when you first get your guineas, try your hardest not to stress them out much as this can trigger ringworm. Its a fungal infection that is zoonotic (can be passed to humans). If you get your guinea pigs from a petshop, look out for crusty noses, nasal discharge, sneezing, raspy breathing (signs of a respiratory infection) and also bald patches on their coats, particularly on their face as this is a sign of ringworm.
Good luck! They are wonderful animals!
I suggest getting girlies
(that way, if you decide you want more
its easy to bond them!)