Player crafters can repair your items, or you can purchase repair deeds and use those in the appropriate shop to repair your items yourself. Ie buy a tailor repair deed, go to a tailor shop and click the deed, target leather item, repaired! Yay!

If you mean the total durability is reaching zero due to several repairs, you'll need Powder of Fortification to raise the overall durability again. Each charge raises the durability by 10 points, and the powder is received as a smithing BOD reward. If you can't get powder yourself, find some to buy from player vendors.