As a novice woodworker I’ve slowly started to amass a small collection of tools, but the ones I use most frequently, at every stage, are my six inch ruler, and my square. Without these two items I would truly be lost. I think the greatest key to successful woodworking is carrying with you at all times the knowledge that you are going to screw something up… if you don’t then hey, good for you. For the rest of us, planning for this inevitability is crucial to your success.
Keeping an obsessive eye on the details and assuring that everything is square, and then doing it another 3-4 times throughout your project will give you a head start. Small mistakes can be repaired along the way or covered up without compromising your finished piece, but if things aren’t truly square you’re going to notice right away, and it’s something you’re going to notice right away every time you look upon your work; until the day you throw it in the fire.