Amanda, building stairs is generally one of the most difficult carpentry jobs that there is. In addition, it sounds like there are some code issues that concern your health and safety. You have to be aware of what your rules/laws are concerning fire protection between the garage and other parts of your house.
I do think that you need to contact a licensed professional for this. You probably will have to have a building permit for this work and your pro should obtain it. If you get a hassle on this point, you should contact your local building inspection people. This might sound like a lot but it could save a LOT more problems for you later on insurance and future sale of your property.