First search the internet for plans. Some sites offer free floor plans while others may charge you. Abettergarage.com has floor plans that will get you started. Look through some of the plans and find the best one for you. You want it to match your house structure as much as possible. check to make sure you are not violating any restrictions that may be enforced in your area.
Print out the blueprints of the garage and take them to a office supply store to have them enlarged. This will make measurements easier to view. Write down all the measurements of wood and supplies you will need.
Take the measurements to your favorite lumber store. Lowes or Home Depot would be good. Have someone figure out how much building material you will need. You can save time by sitting down and figuring this yourself.
You may want to reference other materials to help you get started on building your new garage. Look for a garage plans book or other material at the home improvement store. You can also check the library for more garage building references.
If you know someone that has built a fairly large structure before, ask them for help or advice.