tips and suggestions for a) different thickness metals and b) butt joint

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I am taking an intro welding class and have about 12 hours of hands-on with a stick welder.  All of our practicing so far has been with steel of the same thickness.I am planning a wall-mounted shelf for my garage (hobby) woodshop, to store heavy boards. The shelves will hang from the top ledge of a brick wall and be screwed there into the sill plate, and then be tapcon'd into the brick at several locations.I have 1.5" square steel tube, some 1/4" t-bar to be cut into trapezoid shapes that go beneath the shelves for extra support of their butt joints, and 1/8" thick steel plates drilled for the tapcons.When welding the 1/4" t-bar wedges to the 1/8" thickness square tube, are there any tips you'd care to share? I have 3/32 electrodes. Will they do?P.S. I didn't have a chop saw to cut the metal myself, and ordered it cut into specified lengths from a local metal supply house. For some reason, I was having a difficult time communicating with the guy at the sales desk, despite detailed drawings, so to keep things simple, all the cuts are straight 90 degrees. I would have preferred a miter  joint where the short piece that hangs on the brick ledge mates with the long vertical tube.  That joint is a butt joint.  The tapcons ought to take some of the weight off of that stress point.Any tips on how that top butt joint should be welded? Attached Images