Circular saws (sometimes generically called Skil saws after the popular brand) are about the closest thing to being a carpenter's best friend. They're portable and it only takes a minute to adjust the blade depth or angle. In order to cut accurately, however, the saw blade has to run true without the slightest hint of a wobble.
Some saws have a round arbor and others have a diamond shaped arbor. For this reason, the sawblade comes from the factory with a knockout. If your saw has a diamond shaped arbor you'll need to remove it. The trick is doing this without knocking the blade out of whack.
Here's the easy way to do it. You'll need a bolt with a hex head of about 5/8" and a short section of metal pipe with an i.d. of about 2" to 3". Now it's a simple matter of centering the pipe on the underside of the blade, putting the hex head on the knockout and giving it a smart rap with your framing hammer.