When your string trimmer runs out of line, you can go to the home improvement store and buy a new spool full of trimmer line. But that costs a few bucks. It's much less expensive to buy replacement trimmer line and respool it yourself. It might seem a little tricky at first, but you'll be able to quickly get the hang of it and get back out in your lawn.
Remove the spool from the string trimmer. This will vary between models, but usually entails pushing in a couple of tabs on the spool and pulling the spool out of the housing. Remove any leftover line on the spool.
Place about 1/2 inch of the end of the replacement line into the small hole in the center of the spool. Start to wind it in a clockwise direction, or if there are arrows on the spool, follow that direction. While winding onto the spool, make sure to not overlap the line until the layer you are working on is wound on. This is what creates tangles.
Stop winding when the new line is about 1/4 inch from the top ridge of the spool and cut the new line at that point. Carefully snap or screw the spool back into place. You will probably have to thread the end of the line into a hole in the spool housing.