Excellent question. The nice thing about the answer is everything is readily available at your LHS or Walmart and Home centers.

#1 You need a flat surface that you can push pins into. Do a search here on RCU for "Building Board" You will see everyones preferences, but most are using celotex ceiling tile.

#2 4 foot straigtedge. I would never be without mine. The first use is to check to see if #1 is flat. Then use it to check straightness of everything you are building. Put it on one wingtip and measure back to the tail to make sure your wing is aligned properly for installing wing bolts. I cut all my Ultra cote with it and an exacto knife. You can make stripes, sunbirsts in no time.

#3 Exacto knifes and other cutting tools.

#4 Glues. I prefer elmers carpenter glue for everything except firewall, landing gear mounts and anything related to holding a wing down. For those I use epoxy.

All the rest of the things that Charlie P said. Get lots of cloths pins for clamps. T pins are a must. My favorite pins are the "modelers pins" from Hobby Lobby. They have a longer taper and will never split balsa or any wood. Change your sandpaper often, and as Charlie said - never sand without having your paper on a block (or something flat).

It's really fun and rewarding to take something you built out to the field.