If you get a guinea pig, you need at least two. They're herd animals and need a friend of the same species ^.^
They need a lot of room, they're not a tiny cage animal. Bare minimum is 8 square feet of space for a pair. Bigger is always better.
They're not cheap. They need unlimited hay, each pig needs 1 cup of pig safe vegetables per day, and 1/8 cup of a high quality pellet per day.
Vet costs are expensive. They are exotic animals and need to go to an exotic, cavy-savvy vet. Normal cat and dog vets do not have the expertise to properly care for them. I brought my pig to the vet (spinal injury) and the bill to ran to about $600.
They poop a ton. Their cage needs to be spot cleaned every single day (sometimes twice a day), and will probably need a full cage clean 1x a week (depending on the bedding, but it's a fair estimate.
They will also pee on you.
But they are lovable and full of personality
The perfect little pets in my opinion, despite all the energy, work and money that has to go into them.
Keep in mind - each has their own personality, and not all are lap piggies. I had one girl who loved the lap, would even take naps with me. I had another who couldn't stand to be picked up/on the lap, would tug and tug on my clothes until I put her back. She was a great pig on the floor, she even knew some tricks and was trained with a few commands, but she hated being in the lap.