Follows are simple instructions for putting together a rolling workbench.
Step 1: Get rid of your old desk
Put it on Criagslist and greatly under-price it so that someone will actually buy it.
Step 2: Go get stuff
(x2) 48" x 36" x 3/4 plywood board
(x1) Set of adjustable bench legs
(x4) Locking casters with threaded rods
(x1) Miniwax Water-based Polycrylic
(x1) fine grit sandpaper
(x1) power drill with misc. drill bits
(x20) 1-1/4" wood screws and washers
12" x 48" and/or 18" x 48" plywood board for storage shelves
Misc. nuts, bolts and washers