Is there a hotkey for using your equipped potion?

[Bagmeister]

I stocked up on healing potions and can't figure out how to use them :P

[Raze_Larian]

Click on the Browse Potions button, in the top right, on the right side of the UI at the bottom of the screen. You can use a potion directly from there, or while the mouse cursor is hovering over a potion icon in that menu, press a function button (F1-F10,F12) to assign a hotkey.
You can also use them directly from your inventory by left clicking a potion or stack of potions (same as food: meat and bread restores health, chicken mana).

Individual health and mana potions are more effective at the very start of the game, but once you get up a few levels you'll probably want to switch to restoration potions[www.larian.com].