chisels (gouge, skew,parting tool in my humble opinion) box of 6 rolls of sandpaper (150 to 600), CA glue (medium, to start) for gluing tubes and for CA finish, micro mesh sandpaper if you are doing CA finishes, if you are not doing CA finish, then a friction polish like Mylens, way to cut blanks to length, way to drill blanks (on lathe with chuck, or on drill press), belt sander or end mill to clean up blanks after inserting tube; pen press or some sort of clamp to press pen parts together during assembly; paper towels for apply finish, CA accelerator if doing CA finishes.
There is lots more stuff that makes it easier/faster/better quality, but I think that will get you started. PM if you have more questions. Looking in the IAP library is also excellent. I would also suggest watching You Tube videos on turning, you can learn a lot from them. If you want to spend some money and have a Woodcraft store near you, they also offer good classes for beginners. Good luck and have fun !!