How to use items in Stardew Valley?

Just select the item on your hotbar and click to use the selected item.

Using standard tools - Select the tool in the hotbar and then left-click the space you would like to use it on. This can be a little difficult due to the rules of which space will be selected and distance restrictions. Turning on the 'show tool use location' option can make tools easier to use. Note: If the mouse pointer is a certain distance away from the player, the action will be considered to be taken 'in front of' the player character. This can be confusing at times, especially when fighting, as it may seem like your character suddenly is turning to face a direction or isn't turning. The same behavior will be used for using the action buttons for tools and weapons (c for tool, weapons standard attack, x for weapon's secondary attack)

Using food, consumable items - Select the item and right click (or press the appropriate keyboard key (x?). This will prompt you whether you would like to eat it.

Giving gifts - Select the item in the hot bar, hover mouse over recipient while next to them. The mouse should change to a gift box. Left click on the person. Note: There is no prompt for this action.

Using item-loading world placed tools/devices (furnace, seed maker, barrel, etc): First place the device on the ground somewhere. Then select an appropriate item in your hotbar, click the device while standing next to it. This should load the device with that item.