You need at least level 1 in Repair, then select it as your active skill, right click somewhere onscreen (away from the menu bar or open panes) to turn the cursor into a hammer and anvil symbol, then left click on the equipment to be repaired.
screenshot descriptionYou should eventually be getting most of your mana through mana/intelligence bonuses on equipment, though you can also just eat chicken or use mana or
restoration potions more frequently.