Determine how the space will be used. Will you be building a two-car garage to actually park two cars, or to park one and use the rest of the space for storage? Define what the garage will be used for specifically.
Determine the real size required for your two-car garage. A bare minimum required size for a two-car garage is 18 feet wide by 22 feet long. However, the actual car sizes must be taken into consideration. For people parking a full-sized SUV the size required is considerable greater than for those parking compact cars. Garagehowto.com recommends 24 by 24 feet for two-car garages with ample door-opening room plus some storage.
Decide your budget. According to costhelper.com, a two-car garage of 380 square feet will cost between $13,300 and $17,100 if you have a contractor build it for you. A do-it-yourself garage will run you at least $5,000 for materials.
Choose the materials, keeping in mind how the garage fits with the rest of your home in appearance. If the garage doesn't blend well with the rest of the house, your property value can be reduced.
Consult with professionals. You may want to speak with a designer for your plans, a builder inspector to find out requirements for building permits in your area, and if you are hiring a contractor you'll need estimates from at least a few before choosing one.