hello Lysse, welcome to the world of being wrapped around your rattie's paws LOL
How long have you had your little girl? If it hasn't been long, can you go pick up a friend for her where you got your girl? they are incredibly social animals and you should always have 2 unless there's special circumstances involved.
BUT make sure your cage is big enough for 2 first.
Punch in the dimensions of your cage into this cage calculator and it will tell you how many rats can live comfortably in it.
I always found carefresh dusty myself, and use fleece (washable), aspen and lately I have been using Yesterday's News as a litter in the bottom of some of my cages. I also use it in my litter pans. Yesterday's News is a pelleted paper for cat litter.
Aspen should be quality, not dusty in the bag. and do not use Cedar or Pine as they are dangerous for your pets.
There are many other beddings you can use, soft sorbent, etc, etc but keep away from corn cob bedding as it is too absorbent and can actually draw moisture from your rat's tail causing a condition called ringtail which can make them lose the tail painfully.
One thing to be very aware of, is that ANY wood or paper-based litters (aspen, carefresh, etc) should be frozen for 48 hours before use. Rat mites and lice can sometimes hitch a ride in the bedding and your rats will end up infested. Itchy unhappy ratties, vet visits or at the very least, having to treat your rats and scrub your cages.
Diet should be a quality lab block (Harlan Teklad is considered the one of the best blocks and can be ordered online), Oxbow Regal Rat is superb but some rats will stop eating it completely and will become dangerously thin...kudos to anyone who has ratties who stay on this diet (its the best out there)
Mazuri is fine (Petsmart) but has a high protein level and you want, low fat, low sugar and low protein diet for your rats, as it can keep them healthier longer. Weirdly enough a good block for rats is Living World Extrusion for Hamsters...hehe. There are also mixes you can make up with a quality dog kibble and cereals, etc (for eg. Suebee''s Mix, but even Suebee herself only created this diet because she had no access to quality lab blocks).
Supplement with veggies, a little fruit and even less protein
here is the forbidden foods list
Don't get sucked into yogurt drops being good for them, its pure sugar, try multi-grain cheerios as a treat instead.
As for the sneezing, it sounds like New Home Sneezies...and if there's no other symptoms and your girl doesn't end up sounding congested and wet, then it should pass in a week or so.