To activate Galecaster Colossus’s ability, you may tap any untapped Wizard you control, including one you haven’t controlled continuously since the beginning of your most recent turn, such as itself. (Note that tapping the creature doesn’t use {T} [the tap symbol].)
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.