The purpose of a carport is to keep harsh elements off your car or truck. Not quite a garage, a carport will give a certain degree of protection regardless of outside conditions. It doesn't have to be fancy, and you can build one pretty cheaply if you do it yourself.
Measure out a 365 by 365 cm (12 by 12 foot) square on the ground. The corners will be where your corner posts set. You can make the square on the ground with a stick, or just use rocks or other markers to designate the corners. The most important thing is that you make sure all the sides are even.
Dig your holes to 60 cm (24 inches) deep. Use the post hole digger. Do not dig when it is extremely wet, extremely dry or extremely cold, as all of these things will make digging the hole very difficult.
Plant the 10 by 10 cm (4 by 4-inch) poles inside of the holes. Fill each hole halfway with dirt, then level each pole off on all four sides. This means that you are making sure it is straight up and down as well as level on top. Once level, fill in the rest of the hole and tamp down.
Frame the top of the posts. With your 12 foot 2-by-4s, construct a frame around the top rim of the posts. Flush the frame with the top of your posts. Attach these with your 2-inch screws after drilling holes with your drill. Get a friend to help you hold the posts in place and the ladder steady.
Put the tarp up. At one top corner, drill in a 5 cm (2-inch) screw and leave 0.75 cm (1/2 inch) protruding. At the corner of your tarp, there will be a grommet hole, put it over the protruding screw. At the next top corner, put in another 5 cm (2-inch) screw and allow to protrude 0.75 (1/2 inch). Loop a grommet hole over the screw. Work your way around the top frame, putting in screws and looping grommet holes over them. You want this tarp to be very tight, so adjust the last two screw holes accordingly. Once you are done, you are ready to put your car under cover.
These tarps are heavy duty, and will last for years, but they won't last as long as a conventional roof or fibreglass panels. Plan on replacing your tarp about every 5 years. You might want to stabilise your posts with cement. It will add twenty to forty dollars to the cost of your carport, but it will give you much added stability.
Always work with a partner when you are working on a ladder or with heavy beams.