First, you want to hover the cursor over the tool you want to repair. It will list what items you can use to repair it.



Put one of those items in your inventory. Use the RIGHT mouse button to drag it over the tool. The tool should now have a green outline. Let go of the mouse button and it will use one item to repair your tool a certain percentage depending on which item you used to repair.



You can use a whetstone to repair melee weapons, or oil to repair guns. Same drag and drop with the RMB.



To repair vehicles, walk up to the damaged vehicle with your stone hammer equipped. In the list of actions you can use on the vehicle there will be an option to repair. Use the mouse wheel to scroll through the options until you see repair. Hit the F button on your keyboard.



Hope this helps! Have fun!