Long time cavie owner here! Guinea pigs are VERY social animals who are extremely friendly and talkative with their owners, they take a gentle and calm hand like most rodents and some first time owners can be put off by how skittish they seem, once your guineas get to know you, and you take the time to socialize them with people, they'll come right to you quite happily.
Some critical points:
Bedding and cage changing: Ensure you get good quality bedding for your guineas, the thicker stuff is what mine liked and it was more handy for cleaning as well. Make sure the bedding your getting hasn't been treated with any form of chemicals like "fresh scent" type stuff for cat litter. It can be very detrimental to ANY rodent's health. Your guinea pigs cages NEED to be changed once a week.
Social: As mentioned above and correctly noted by Welu guinea pigs function best in pairs or more. Make sure you have only girls or only boys unless you want lil'uns. Unlike say ferrets guineas do not need the same high level of interaction, some strokes and pets and a treat or two while topping off their water, food and hay for the day is fine, but more is usually welcome as well. This is especially true if you have multiple guinea pigs. If you have only one, you will need to ensure it gets regular, detailed attention every day. Considering how cheap cavies are ($20 at my local pet store) it's worth it for the happiness of your animals to go ahead and pick up a second or third)
Chewing: Guineas, like most rodents have those big ol'teeth and need the opportunity to chew on things to stay healthy. Salt chews are sometimes recommended but we found that these chew sticks that look like great big fruit and nut bars that clip to the side of the cage are best.
Chow: Good quality guinea feed is important, if possible get feed specifically formulated for guinea pigs, a mix of pellets and seeds. This should be supplemented by hay, and vitamin C yoghurt drop treats which should be available at your local pet store.
A retreat: Like most animals guinea pigs need a place they can retreat to within their enclosure, a little plastic hutch does the trick usually. If they retreat don't pull them out. Let them come to you.
Talking: Guinea pigs talk, a lot. At all hours of the night and day. (Guinea pigs can be active for up to 20 hours a day.) They are very communicative and expressive about their likes and dislikes and as you get experienced you'll find they do actually "talk" to you. Including greeting their favored person when they come into the room. (My guinea pigs made different noises when I came into the room or when my mom did compared to everyone else)
It's definitely something to consider, especially if you're going to be living in an apartment.
