Purchase a circle template from your local arts and crafts store. These come in a variety of sizes, so make sure you read the package information. Most circle templates are between $5 and $10 and can be purchased at most art or office supply stores. This tool is very useful for people who want to draw a circle when the diameter is known.
The flexible curve tool is another useful drawing utensil that you can purchase at most office supply stores. They are much like curved rulers, but they can be manipulated to bend into any shape. Depending on the size you get, you could create a circle with an 18-inch diameter. If you are trying to create a circle based on diameter instead of circumference, then this tool is not very useful.
A compass is another very useful tool for drawing circles. A compass can also be purchased at any office supply store, as well as many discount retail stores. To use it, you will need to know the radius of the circle you'd like to draw and adjust the compass accordingly. Place the center peg of the compass on your paper and twist it a full 360 degrees with the pencil part of the compass touching the paper.
A ruler is another good tool for drawing circles, especially if your circle is very large. Put the ruler on your paper and draw a straight line the length of the diameter of the circle. Make sure to mark the halfway point. Then rotate the ruler in any direction, and draw another line the same length, but make sure the halfway point lines up exactly on top of the halfway point from the previous drawing. Continue this process all the way around the circle, and then connect the ends of the lines with a curve.