What you are proposing is a serious piece of construction, the specifics of which are almost impossible to address without seeing your house. A few general pointers though, from someone who has done this a couple of times :-)
Your first and foremost concern will be structure. Will your current weight bearing structural members be able to support the additional weight; including your foundation, beams, piers/posts (if a crawl space) and weight bearing walls? A "no" answer here will make the proposed conversion much too expensive to be practical. The person best able to speak to this will be a residential structural engineer. On a project of this size and scope, his or her structural assessment is the first thing I would get, regardless of who does the work.
To make an attic a living space, it must have minimum dimensions of (if I recall correctly), 70 sq feet with a head clearance of 84 inches, with a minimum of 7 feet as one side of this 70 sq feet. HVAC, electrical and plumbing (if applicable) must also be planned for.
There are a lot of code requirements for building a stairway and railing, as well as constructing dormers, insulating, knee-wall construction, etc, etc. The one who should be the most familiar with code construction requirements will be a licensed contractor.
The other alternative is to do what I and undoubtably several others on this board have done......take the time and figure it all out yourselves. Its not rocket science and it is very doable; but it will take time and research (including several how-to books from the library), a good collection of tools and probably the most important thing, it will require motivation and a love of doing it. IMO, this means that you must look forward to doing the work and it will occupy much of your waking thoughts.
Hope you have a great time on this project. Best wishes