In addition to the excellent information that AnaMusic has given you links for, you might download from the Internet OnmiDiskSweeper (free) and open it. It will show all of your files and the respective sizes. This can aid you selecting ones that you may not need and thus deleting then.
An easy way of determining if you actually have a storage issue is to click on the HDD icon on your desktop COMMAND I (as in the letter i). This will generate a display showing the actual space used and what is still available. Ex.