I'm fairly skilled with an angle grinder but I don't know how you are. I would lay out the measurements for the door, then slap a cutting wheel on my grinder and cut away. Remember to check and make sure you're not cutting through wiring and such sammiched between the wood and steel.
Cut lightly so as to just cut the metal. Then use a jigsaw/recip saw or even circular and cut the wood that is behind the steel.
Try and make your cuts as straight as possible because the material u cut out will be reused as your door.
Then look at how trailer doors are made, they utilize an aluminum trim with weatherstripping around the perimeter of the door. Now you have a door, and a method for sealing. All that is left is hardware (hinges and a latch/locking system).
You will probably want to do the passenger side so you can easily load from the sidewalk or unload/access from the side of the road safely. I would also put the hinges toward the front of the trailer so that if the latch fails, or the door is not shut all the way, it won't fly open under speed and slam into the side of the trailer.
Questions?