@mauvehed
D&D is not a survival horror game. Goblins and kobolds and stuff should be trivially easy to kill. Give big obvious hints on when to fight and when to run away. Stick to easy, straightforward classes and races that you understand after reading the book.
Keep the adventure simple and trope-ish. You're in a tavern. A mysterious stranger asks for help on a quest with some sob story, and will pay you 30 gold, half now, half later. The quest leads to a cave in the woods just outside of town. Kill some baddies with the new friend through acts of derring-do (or stealth it with a Rogue or something). Collect cool magical treasure. Play with it, figure out what it does. Bask in glory of a job well done...
SHOCKING REVEAL: the stranger was a bad guy the whole time! They will definitely overpower the PC if they stand and fight, so they should either get their ass beat (but not killed) or the PC runs away. The stranger disappears, and now is the villain for the rest of the campaign.