Draw a rough sketch of the shape of your garage floor from a top-down view.
Divide your drawing into as few rectangles as possible, so that the drawing is composed entirely of side-by-side rectangles. If there are any angled walls in your garage, divide them off into right triangles, and then divide the remaining space into rectangles.
Measure the length of each of the sides of each rectangle in your garage. Write the length of each side next to the corresponding side in your drawing. If there are any triangles in your drawing, then measure their sides as well. Make all measurements in feet.
Multiply the length of one side of a rectangle by one of its adjacent sides (not the side on the opposite side of the rectangle). Write the result in the middle of the rectangle. Repeat for each rectangle in your drawing. If you have any triangles, multiply the sides adjacent to the right angle (the sides that aren't one of your garage walls) by each other and then divide the result by two. Write that result inside the triangle.
Add up all of the numbers inside the rectangles and triangles. Now you have your garage's square footage.