This Instructable will show you how to build a corner shelf for little money and it takes little time.
Step 1: What you will need
drill, level, stud finder, drill bits, screws ( not pictured: square)
wood: 1x8 board and 1x3 board( I ripped a 1x6)
cut the 1x8 so it will fit in the corner( 45 degree angle)
Step 2: Support boards
these boards have to be sturdy so make sure you find the studs first. i like this support system better than brackets. predrill the holes so as not to split the boards.
Step 3: Place your shelf boards on top and repeat for more shelves!
you can put a little nail in the ends of the shelf boards to secure them to the support boards. the door and window frames keep my shelves in place pretty well.