I would like to preface by saying that I have built a few skate ramps and tree forts in the past, but this is my first foray into furniture design and construction, so YMMV
Step 1: Materials & Tools
4 - 1x10 boards @ 10' long
1 - 1x6 boards @ 8' long
1 - box of #8 1 1/2" wood screws (75 were in my box)
12 or more 2" finishing nails
Sandpaper in varying grits 80,120,240,320
wood filler or plugs (I used filler, but in the future I think I'll try plugs) or you could just leave the screws exposed if you wanted to?
Pre-stain wood conditioner
Loud music to keep you motivated.
Speed square (got mine for 3.99, best tool ever)
Saw ( i used a chop saw, but pretty much any wood cutting saw will work I suppose)
#8 Countersink drill bit (if you plan to countersink your screws)
Regular 5/32" drill bit
Electric sander (unless you want to sand by hand for days, or leave it kind of rough and rustic?)
Brushes/rags for your stain/finish
Total cost for this should be around $100 or less. I ended up spending ~ $140, but I bought more boards for another project, and way WAY too much pre-stain, stain, and poly ( I have enough for 2 more of these shelves...).