I wouldn't buy from a pet shop.
Pet shop animals often die sooner because they are sick, they have inbred diseases and are rarely ever looked after properly in pet stores.
Buying from a pet shop is promoting irresponsible breeding (Just like in dogs, so it is with all other animals in pet stores) And they don't know how to sex small animals very well at all.
You are likely to ask for 2 females, and get a female and a male, and be stuck with a litter of babies. Or, you may end up with 2 males who fight. Or, get the 2 females, but one or both may be pregnant. This is VERY common when getting pet store rodents/rabbits/cavies.
Check rescues around you, they often have mice.
For cage I would buy a 10-20 gallon aquarium, and a screen lid for it.
I have a question though, you say your parents aren't helping; but they know you are getting the mice right?
May I ask what happens in an emergency? What if your mouse is suddenly very sick and, needs $200.00 surgery to survive. Just a theory. How would you get the money for that?
Vet bills in all animals can get quite expensive, and if you're having trouble with the basics you need to figure out a way to pay vet bills should the need arise, beforehand.
For bedding, do *not* use pine or cedar, they are toxic.
Aspen and carefresh are terrific beddings (I prefer carefresh)
Two females would be the best choice.
A healthy mouse diet consists of carbs, veggies and proteins.
Protein:barley, wheat, peas, oats, corn,dog biscuits (No onion or garlic in the ingredients!)
Carbs: yeast, milk, etc.....
Do research on safe/unsafe mice foods....Add these sorts of foods to your mouse's regular "mouse mix" bought from the pet store.
Here is an article on a good diet:
http://www.bellaonline.com/articles/art9805.asp