I chuckle at people who say they built their own home, when all they really did was walk into the office of a neighborhood builder and fill out a specification form. But that is what some people consider "building themselves". After all, there wasn't a structure there until they got involved, and now there's a structure there. So okay.
If that is all you want to do when you build your garage, this site can help you throughout the planning and building process by helping you to make decisions and verify that the work is being done properly.
There are a few basic levels involved in construction, and you can choose to be any or all of them:
Owner: Works out the specifications with a builder and checks on work to be sure it's being completed properly and on schedule. Might also handle permits/blueprints if the builder doesn't handle them.
General Contractor: The main builder who the owner works with. Many of you will choose to be the General Contractor as well as the Owner. In this case, you are responsible for hiring subcontractors to complete sections of the project, such as excavating, foundation, framing, finishing, roofing, doors, etc... You are also responsible for obtaining blueprints and permits and making sure all inspections are performed.
Subcontractors: Subs handle particular pieces of the project, as stated above. You may have an excavating company that prepares the site, a concrete company that builds the foundation, and other subs to handle the actual framing and finishing. If you are competent in a particular area you may choose to handle some of this work yourself. For instance some people like to do the siding and/or trim work themselves.
OR: You can do what I chose to do, EVERYTHING!
That's not entirely true though...
I had a subcontractor install the shingles, ice shield, and tar paper for the roof. It would have taken me weeks to do what they could do in a day.
I had an engineer create the blueprints, which served as my instructions, basically, and were also needed for the permit.
I had a concrete company deliver the concrete to finish my foundation using a mixer truck with a concrete conveyor belt, and the driver helped with getting the concrete where it needed to go.
Everything else I did either myself or with my wife's help.