There are many ways of adding a shelf. This is a quick, industrial looking one that has some good strength.

Step 1: Materials

Pipe parts found at Home Depot in the plumbing isle.
2x 1/2 x 6 inch galvanized threaded Pipe Nipple
2x 1/2 inch galvanized Cap
2x 1/2 inch galvanized Floor Flange
Screws sufficient enough to hold your shelf and the weight of the items you will place on it. (I used 1.5 inch drywall screws.)
1x 2x6 untreated board cut to your preferred length for free or you can cut it at home. I got an 8 footer and had them cut it in half to make two shelves. (Also fit in my car better)

Step 2: Gather Tools

Drill with drill bits and screwdriver bit
Screws
Level
Pencil
Stud Finder

Choose location.
Use Stud Finder to find wall studs at width you want to use for mounts. (I skipped a stud)
Mark stud center using your stud finder and mark the right side of the studd and then left side. The middle of these two marks will get you the center.
Mark hole of Floor Plate top hole only to correspond with your stud center and desired shelf height. This will be used with screws to screw into stud.
Use level and align with mark for first mount hole and second stud center location. Mark to locate height of second mount. Mark second mount top hole only.
Predrill both screw holes you have marked with drill bit smaller than mounting screws. (This is optional, but a good idea to always predrill holes into wood)
Screw Floor Plate into wall using top hole and leave loose. Gravity will level this Floor Plate for you, and you can now drill bottom hole into stud and screw it in.
Tighten both top and bottom screws.

Screw Pipe Nipple into Floor Plate, leaving loose
Screw Cap onto Pipe Nipple leaving loose.
Place 2x6 onto wall mount and adjust tightness of parts as needed to ensure that the 2x6 is laying flush onto the Pipe Nipple and not crooked due to laying the Cap edge nor Floor Plate edge.
Center 2x6 on wall mount