Grab the clutch pedal with your hand and yank UP towards you firmly. You should hear a zip tie sort of noise, this is the stock mechanism adjusting itself and taking up any slack in the cable.
Next you will want to probably have the fluid changed if it never has been, ESPECIALLY if you are an 01-04 car. The factory overfilled those and over time being overfilled will cause them to fail. Correct fill is 3.2qts (factory was 3.8 and has since been changed to 3.2) and a real good readily available fluid is Pennzoil Synchromesh which you can get at any Autozone.
If adjusting the cable and changing the fluid doesn't fix it and you are not mechanically inclined... going to have to go to a shop. Could be something as a new clutch cable, could be as bad as a trans rebuild.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.