Go to 'Inventory' (or press 'I' on map screen) and put food in the food box on the lower right of the screen to the left of your inventory.
Seeing as we're asking newbie questions, I have one:
Can you switch to the weapons in the 2 extra weapons slots without going back to your chest in mid-battle? I registered the game, but I don't think I got a basic manual, and this issue isn't raised in the manual section of this forum. I think I've got everything else pretty much figured out but this.
Only ask coz I've seen other characters switch weapons (most often between ranged and melee weapons) on the fly.