I have just lost my two guinea pigs to old age.
My advice would be:
Don't just get one guinea pig - they are social animals and it would be best to get either two or three of the same sex. Keeping two piggies is no more work than keeping one.
Keep indoors in a cage if possible. It's unpleasant for them to live outdoors in the winter especially. They're also more part of the family if they live indoors.
Ensure that they're kept dry and out of drafts - damp is bad for them.
Always buy a premium quality guinea pig food.
Always keep a good supply of fresh water for them.
Handle carefully - sudden drops can kill them.
They need modest exercise and fresh air - on a lawn if possible.
Don't bed them down on straw - use hay instead, but always keep them clean as hay can develop mould spores.
They are prone to teeth overgrowth and urinary tract infections. Prevent these occuring by providing lots of nibble sticks/snacks and giving dandelion leaves each week along with loads of fresh water.
Be prepared to clean them out about twice a week.
Don't place them onto sawdust. Place a layer of sawshavings onto their cage, then cover with newspaper, then cover that with a good layer of hay. Sawshavings are too irritant to their respiratory passages.
They like: apple, carrot, spring greens, broccoli, sprouts, grass and grapes. Don't give too much fruit as the sugar can lead to diabetes.
Make sure you have them sexed properly in the store.
Guinea pigs are friendly, gentle souls and make fantastic pets.