Carefully read and follow the instructions that came with your circular saw. If the user manual differs from the instructions that you read here, follow the instructions in the user's manual.
Before you begin to tighten the blade on your circular saw, always make sure the saw is unplugged. Your saw should have come with a spanner or Allen key. Use this or a socket wrench to tighten the arbor nut, which holds the blade in place.
If your saw has a shaft lock, lock the blade in place so it won't move while you turn the arbor nut. If the saw doesn't have a shaft lock, use a clamp or vise grips to hold the blade in place while you tighten the nut.
If you are unsure which way to turn the nut, this is the rule: The nut always tightens in the opposite direction of the saw blade's rotation. Otherwise, every time you used your saw, the blade's rotation would unscrew the bolt, and the blade would fall off! In general, the blades on right-handed circular saws turn counterclockwise, so you'd tighten the bolt clockwise; the blades on left-handed circular saws turn clockwise, so you'd tighten the bolt counterclockwise.
Hold the outer flange (rim) of the nut with a wrench to keep it from turning. With your other hand, use the spanner, Allen key or socket wrench to turn the nut in the opposite direction that the blade spins.
Check the blade on your circular saw periodically to make sure it's aligned and tightened properly. Poorly maintained power tools are a cause of many preventable accidents. Never force a power tool. If you have trouble tightening the saw blade yourself, take it to a professional for servicing.