How can I cut angled support braces to support a shelf in my garage?

I need to know the angles, and the length.

They sell all sorts of steel shelf brackets at Home Depot or Lowes. Some of them are very strong.

If you want to cut them yourself then just whatever angle and length works best with your shelf. There is no rule that it has to be a certain length or angle.

measusre the width of the shelf and cut your support that long to go right under the shelf. Then measure from the back of the shelf down, remember to add in the support width and cut that side down the length you want it.=======

That depends on the weight the shelf needs to hold.

For light to medium shelves, I just use a piece the same width as the shelf and cut 2 45* pieces which I fasten to the bottom of the shelf. (ex. for 1X8X6 use 8' board, mark end at 45* and cut then mark square and cut, then cut remaining piece to length. Use glue and wood screws to hold the 2 triangles to the bottom of shelf, use a form of hanging bracket to mount shelf to wall.)

Hacksaw

If you are making the supports out of wood,,,this is an easy way to do that,,,,put a 2/4 under the self ,,,run a 2/4 down the wall under the 2/4 on the shelf the length you want it,,,hold another 2/4 at the end of the shelf beside the 2/4 and mark a line across,,,at the 2/4 on the wall at the bottom do the same thing make a mark across the 2/4 you want to cut ,,,the 2/4 you want to cut has to be long enough to reach the to ends of each 2/4 you have already attached take that measurement so the 2/4 you want to cut will be long enough,,,make sure the shelf is square to the wall,or if not on the wall still make sure it is at 90 degrees