Aside from proper footwear, keep in mind that trail miles and road miles are not the same. You will use a lot more of your lateral stabilizing muscles because of the uneven terrain. It is very easy to get injured if you jump in with both feet. Pun intended.

I would suggest that you start with short runs, regardless of your current road mileage to give your muscles, tendons and ligaments a chance to adjust to the demands of trail running.

Also remember that you will be slower, so fuel and water will become an issue sooner than on the road.

Nothing else jumps to mind right this second aside from welcome to the dark side....