A key decision to make prior to embarking on building the BOM is what framing system you want to use. 16" on center or 24" on center studs? For most garage needs 24" is fine and uses less lumber. Check your local codes, as you may not be able to have a 2nd story above the garage with 24" framing. 2x4 or 2x6 framing? If you plan to have a heated garage and live in a cold part of the country, go 2x6 and insulate.
So I took my building sketch to the Menards design center kiosk and answered a series of questions about the building I wanted and viola I had a recommended bill of materials in under 5 minutes. I would recommend going over this print out manually to add, remove, or substitute items. For example, you may want to use a pneumatic nailer (recommended!) for framing/roofing and thus would not need to buy as many regular nails as the print out suggests. Additionally, you may want to add insulation, space your studs to 24", use OSB (oriented strand board) instead of plywood, or specify a different style of window. The key thing that the print out gives you is an idea of the number of pieces needed, and the parts that you might miss such as drip edge, drip cap, shingle starter strip, etc that you will need. It is also a good idea to add a few extra pieces of lumber such as 2x4's and 2x6's in case you measure once and need to cut twice.
You will also need to be aware of building codes when ordering, although often the print out from the kiosk will take many of these issues into account. For example, the bottom plate on your framing is in contact with concrete and thus must be treated to prevent rot. Also, when sheathing your roof, you need to use H-clips between the trusses on the horizontal seams. Snow and ice shield membrane may also be required along the lower portions of the roof. These little details can be anticipated if you spend some time reading the building code and chewing the fat with your experienced construction buddies. The International Residential Code is maintained by the ICC (International Code Council) and serves as the basis for the home building codes in most municipalities. You can buy a copy of the code from the ICC website (click here) and while comprehensive it can be tough to digest.
You don't need to make all the BOM decisions at once and may need to mix and match suppliers to get the right materials. For example, in order to match the shingles on my existing house I had to order from a specialty roofing supply house. The key will be to get all the critical parts on the building site at the same time so that you can effectively use your beer-paid volunteers and get to a stable work point (i.e. the roof shingled) in case you need to take a temporary vacation from your project. Details such as what lights and color siding can wait a bit. Once you have finalized the BOM, place the order and arrange for delivery. Even the largest suburban command vehicle will have trouble hauling a dozen roof trusses and 50 sheets of plywood so shell out the $50 for delivery.
The following is a sample bill of materials for a 20' by 22' garage with 5/12 roof pitch. This is not an exhaustive list (I must have run to the store about a hundred times to get something I forgot), but the major components are there. All dimensional lumber is #2 grade or better.
Qty Item Unit Cost Subtotal 75 2x4x8' Stud 2.18 163.5 20 2x4x12' 2.95 59 10 2x4x16' 4.58 45.8 6 2x6x10' 3.99 23.94 2 2x12x18' 20.49 40.98 12 1x6x12' Quality grade 6.75 81 8 2x4x12' AC2 treated 4.59 36.72 45 1/2" 4'x8' exterior grade OSB 7.49 337.05 14 5/8" 4'x8' exterior grade fire rated drywall 9.44 132.16 12 1/2" 4'x8' BC grade plywood 12 144 10 5/12 20' Residential 60# Truss 30.25 302.5 20 Rafter tie hangers 0.63 12.6 2 5/12 20' Residential end frame truss 38.21 76.42 2 Roll of 15# felt roofing felt 13.25 26.5 2 Roll of 3' wide ice and water seal 25.85 51.7 2 Roll shingle starter strip 10.48 20.96 3 bags 1/2" H-clips 2.24 6.72 18 Bundles 3 tab 30 year shingles 20 360 2 3.5x50' roll sill seal foam 4.18 8.36 1 16' insulated sectional garage door 439 439 1 Garage door opener 179 179 1 Set vinyl garage door stop trim ~32' 23 23 1 Set vinyl garage door jamb ~32' 52 52 1 Set vinyl garage door brick mode ~32' 32 32 1 Set garage track hanging kit 16.98 16.98 11 10' D style roof edge 2.46 27.06 3 R11 3.5x23x70' kraft faced insulation 31.99 95.97 2 30x42 vinyl windows 107 214 1 5lb box of joist hanger nails 6.79 6.79 1 32x80 Prehung exterior door 153 153 1 Deadbolt and doorknob kit 55.92 55.92 4 6"x12' Aluminum fascia 11.79 47.16 2 16"x12' vented aluminum soffit 13.68 27.36 4 12' soffit frieze 6.99 27.96 4 1-1/4"x10' dripcap 2.38 9.52 1 Large box 16D size nails for pneumatic nailer 45 45 1 Large box 8D size nails for pneumatic nailer 40 40 2 5 lb box of galv roofing nails 4.28 8.56 1 Box 40' Ridge-vent 95 95 2 Box roofing staples for hammer-tacker 6.19 12.38 Total 3537.57Whew! The above will get your garage framed, sheathed, roofed, and the windows and doors on. It is up to you to make the decisions on finishing touches like siding, accent lights, etc.
If possible try and arrange delivery of your building materials after the foundation has been poured and is cured enough to build on. That way you can get your materials delivered right next to or on the slab for convenience. I had to hand carry each and every piece around to the back, which was a pain. Sometimes an "alley" delivery will cost extra, depending on your job site layout. Talk with the driver and if you can float him a few bucks you may have luck getting your materials dropped wherever you want. Those guys are magicians with those forklifts.