Kind of a lot involved with this question…so many different answers.
If your video of the fire is against a solid black background, you can place your fire video on a layer ABOVE your background video and set the transfer mode to screen. This will drop out all the black and the fire will show through. This assumes that both the fire and the background video were shot with the same perspective. Otherwise, adding the corner pin effect to the fire layer may help with some of the perspective.
Now, if your fire is just plain footage of a fire (like at a campground), then you’re going to have to rotoscope the fire out of the fire video and then place it on top of the background video. That is going to be VERY difficult and take a LOT of time. Definitely NOT the way you want to do it.
You can also buy some stock footage from different websites and possibly find some pre-matted fire footage that would work for you. A Google search should help with that.
Either way, you’ve got a lot of options.
Good Luck,
Jacob