Hello and welcome to the forum! There are a lot of necessities but here are a few of the big ones:
1. A good dustpan, guinea pigs poop CONSTANTLY. I clean my guinea pigs cage at least three times a day. A mini handheld vacuum is even better.
2. Guinea pig safe cleaning solution and paper towels. If you let your guinea pigs out, they tend to pee and poop on the floor so some cleaning spray and paper towels are needed to clean up their accidents.
3. I’m sure you already know that guinea pigs need hay constantly. I have a large bucket that I keep beside their cage for easy refills.
4. A trash can. When you have guinea pigs, they make a lot of mess so a trash can (with a lid) can beside the cage is really helpful.
Guinea pigs also need regular
nail trimmings. You could ask your parents or guardian to hold the guinea pig, while you cut their nails.
There are a lot more things that are helpful, but these are the things that I use.
What cage are you planning on using? What type of bedding? oh, and good luck on your guinea pig journey!