Building your own desk can range from super easy ("buy 4 IKEA legs and pick out a table top") to more advanced DIY projects requiring woodworking skills and proper tools. In either case and everything inbetween, half the work is done when you've got all your resources planned out...
Sketchup: plan things out ahead of time and you're less likely to find yourself facing surprises when constructing a piece of furniture from scratch or even when just accessorizing an existing piece. So whether it's factoring in the dimensions of the desk in relation to the room it will occupy or actually creating completely new plans for construction, it's good to sketch something out. But why not "sketch" it out in 3D, just to be safe?
Sketchup is a very popular 3D modelling program for Windows XP/Vista/7 & Mac OS X (10.5+), designed with "3D for everyone" in mind. Unlike most 3D applications the learning curve is gentle and immediately rewarding, as revealed by the plentitude of desk projects shared online created with Sketchup.
Ana White Homemaker: Alaskan DIY blogger Ana White offers hundreds of free plans (as PDFs), like the desk above inspired by the uber-popular West Elm Parsons desk, complete with dimensions, shopping list, costs, and 3D instructions with various angled views for easier construction. Other more traditional desk plans can be found at woodworking sites like Renovation Headquarters.
CableOrganizer.com: So you've got your desk finished, and now want to fit it with cable and wire management solutions to keep the tech mess out of sight? My favorite online retailer specializing in desk organizing and management solutions like grommets, cable covers, pop-up outlets, under-the-desk cable organizers, and other cord & cable tools is CableOrganizer.com. Their prices are usually competitive with the lowest online prices and they carry name brands like Belkin, Bluelounge, 3M, Flatwire, and Lutron. I also recommend keeping an eye out on sales for items like this Wiremate Cord Organizer, which is currently $16.99 vs. $21.95 at Amazon.
HairpinLegs.com: Maybe all you really need is a set of new legs for a tabletop or reclaimed door desk. If you steer more toward the midcentury modern style and/or like an airy-light presence, hairpin legs can make for an excellent option. Available in various lengths (4"-28") and raw or stainless steel finishes. Just note, it's advised to use hairpin legs with a third rod design for stability when outfitting a desk.
Instructables: with a vibrant community of readers and makers ranging from downright geeky to polished chic modernists, the guides over at Instructables offer a wealth of how-to knowledge related to putting together desks of all styles.
Interested in putting together your own treadmill desk? Here's a project! Want to build a desk using a reclaimed shipping pallet with glass top? Of course they have a how-to! There are even plans instructing how to build a desk hanging from the ceiling! And be sure to always check out the readers' comments, which often reveal additional information and tips related to the original project presented.
(Images: as linked above)