You invoke a magical item's power. This requires no roll. However, you can only have one magical item activated at a time. When you choose to Activate a Magical Item, any other item you were using that requires activation immediately shuts down.
You do not need to be a spellcaster to activate a magical item. Consumable items, including wands, scrolls or staves, do not require activation. Magical weapons also do not require activation, but rather grant their benefits whenever they are used to make an attack.