First thing I would suggest is to take a look at our guinea pigs care packet, here -- https://www.nwichinchillas.com/guinea-pig-documents -- and I will elaborate on some additional things as well:
They do tend to be (and tend to remain) flighty animals, as far as when you reach in the cage... and this seems to be the case, no matter how much you handle them. They can be LESS flighty / terrified over time, but it's rare to just be able tor reach in the cage and pet one (to be fair... this is true of most small animals). Once you pick them up and hold them, most just melt and you can pretty much hold them forever. That's what makes them so loveable!
They drink a LOT of water. 16 ounces, for a guinea pig or two, is probably a couple days. We use 32 oz. water bottles on our guinea pig cages, and I feel like I spend half my life filling them (course, I also have a bunch here at the rescue). Definitely would advise to go bigger on water bottles, as small ones will be a pain to keep filled, right from the get go, and definitely have a spare or two, so you can take one off and wash, and still have another one on there for them to drink (or swap them out, or however you'd like).
They also eat a lot of veggies... about a cup per pig per day. That adds up quickly, so stock up! If your family eats a lot of veggies, this may not be a big deal, as you'll already have them in the home, but if not, you will be stocking up weekly at the store, as the stuff they eat isn't all that long lasting, especially the leafy type stuff.
Because of the water intake, and the veggie intake (which contain a lot of water), they pee and poo, a lot. The poo is relatively dry, but obviously, pee is pee, and we change our guinea pig cages about 2x per week. You can definitely tell when they need to be cleaned, as they can get icky quick! Obviously, the bigger the cage, the less frequent you will have to clean it... if you go with a 30 x 18 cage (which would be similarly sized to what we keep our piggies in), it's going to need the weekly cleaning (if not 2x... depends on your preference as well), but if you move up to a larger cage, you might be able to go 1.5-2 weeks between cleanings, and potentially even longer if you spot clean and just remove the soiled areas. I think the cleaning, for the guinea pigs, is what takes awhile.... probably no longer than say cleaning a rabbit, but I'm used to chinchillas, and they can go a week... so to me, it seems the guinea pigs get dirty quick!
They make all sorts of cute noises! Unless those noises are irritating to you, and then... they're less cute. Haha. They can easily learn what the sound of the veggie bag is, or scooping pellets, or whatnot. For us, we use metal coffee cans to haul around the food and scoop from those to feed the animals. The guinea pigs here know EXACTLY what the sound is when we scoop from our feed bins into those coffee cans and start wheeking away! Basically, it's one of those "I want something!!" noises. It's usually cute. Usually.
They do like to hide, so some sort of hide or igloo is good to have for them... if they get spooked, the first place they run is in their hide, so for security purposes and their wellbeing, there should be a hide.
They need their nails trimmed. I can't tell you how many guinea pigs I get in with horrible nails, simply because people didn't know it had to be done. Just periodically, like trimming a dog's nails.
I think that's the main stuff... definitely take a look at the care packet and let me know if you have any additional questions.