If its a normal weapon you can repair it at one of the "repairers" Charsi, Fara (is that her name? I dunno) etc... There is one in each act and you get to them by trading with them and they will have a little icon that saws repair. Click on it and you will have to pay money to repair your items. If its and ethereal item, then your out of luck, once their durability hits zero they are gone for good
2.) Go to the nearest blacksmith and have it repaired.
Since I'm guessing you're brand new to the game, go to town, go to the North East corner, and talk to Charsi. Go to the trade/repair menu, and click on repair and slect you weapon, or just click on repair all and save yourself some time.
Wouldn't kill you to give the users manual a quick once-over either.