How do I select the right size drill when I want to pre-drill a hole for a wood screw?
First you need to decide what you want the screw to do. Below is a picture of a wood screw
If you want to just pre-drill a hole to put a screw in the wall to hold a picture you'll want to select a drill that is about the same diameter as the body of the screw minus the fins of the screw (as shown below)
This allows the fins of the screw to grip the wood without splitting it.
If your purpose is to join to boards together you will want to drill two different size holes. The first hole would be drilled with a drill sized exactly like the one above. The piece of wood that will have the head of the screw in its final resting place should be redrilled with a larger drill. This drill is sized to allow the drill to slip through the hole and only grab the second board. The resultant affect is the cinching of the two boards together. The picture below shows the proper select of this type of drill bit. Note that the threads can't be seen as you view the thickest portion of the drill bit.
Another helpful wood join technique is to clamp the two boards together before you put in the screw. This takes some of the work out of the effort for the screw. If you are using a wood screw like the one shown above in our examples you can also use what is called a "rose bit" to allow for the counter sinking of the screw head.