Decide what's on your wish list. For instance, if you intend to work in your garage, it should have electrical outlets. Furthermore, if you live in a cooler region, you might consider heating your garage to make it useful in the winter. You might also consider adding an extra door so that you can enter it without raising the garage door.
Check your local building code for restrictions before you proceed with your planning. Pay special attention to local rules governing setbacks, because that is one factor that causes the most frequent problem. A setback is the minimum distance between the proposed building and the lot line, and it can vary from one jurisdiction to another.
Decide if you want to hire a contractor to do all the work. Or you can subcontract parts of the job and do some of the work yourself if you want to save some money.
Size makes a difference, so if you have any doubt about how large your garage should be, plan on it being larger. The minimum size of a two-car garage is 22 feet long and 12 feet wide. If you want to be able to open your car doors wide, make it larger. Also, understand that cars vary in both length and width, so you don't want to have a garage that's too small to accommodate them. And if you later replace your cars with larger ones, you will want your garage to be large enough.
Call in an architect to create the plans that will be followed to build your garage. The architect will design a garage that complements your home, which will add value to your property. For example, the garage should have a similar roof to the one on the house, and it should have common elements like doors and windows.
Talk with your mortgage lender about your options to pay for your garage. You can either refinance your mortgage and add to it the cost of the garage, or you can apply for a home equity loan or a home equity line of credit which is also a second mortgage. Either way, it is a good idea to make financial arrangements now rather than having to do them in haste.