Generally (a couple that I've owned), there is a push detent so that you can remove the spool, or spool and some sort of cover. Clean the dirt, grass, etc., out of things, so the "tap" advance will work.
Get the right size "string." The spools on the ones I have owned have either a notch or small hole to retain one end of the string. Then, fill the spool with string. There is usually an arrow to indicate which way the string is to be wound.
Snap the spool/cover back in, threading the string out through the exit port. Pull enough out -- maybe 6 -8 " to get things started.
Start trimmer. Tap on ground or other firm surface to cause the string to feed. There is usually a knife to cut it to length