While a few workmen have developed the skill to manually sharpen a circular saw blade, most of those with experience will advise you to use a circular saw blade sharpener.
Standard Saw Blade Sharpening
To sharpen a standard non-carbide circular saw blade, you'll need a small triangular metal file and a steel vice. Secure the saw blade in the vice. As you sharpen each blade tooth, avoid changing the tooth's bevel angle.
Carbide-Tipped Blade Sharpening
You should only sharpen a carbide-tipped blade with a blade sharpening machine. With the blade held secure, use the machine to hone only the leading tip faces, not the tip sides.