Can I heat treat with just a torch? Do I need a forge?

You can, but it’s not ideal. It might yield acceptable—again, not ideal—results if you’ve got a simple steel like 1080 or 1084. The issue is two-fold: 1) trying to heat the blade evenly, and 2) letting the blade soak in that temperature for a duration of time.

Instead, you can create a cheap 2-brick forge, Soup Can forge, or even heat up a pit of coals. These techniques won’t cost much, but will give you a much better and harder knife in the end.

Alternatively, and perhaps easiest, would be to take your knife in to a shop to get professionally heat treated.

Good luck!