I know of no shelf brackets that would hold up a shelf wide enough for a person to sleep on. There are tons of "how to build a loft bed" sites on the net. I suggest looking at a few.

So I want a loft bed in my room, and my mom was thinking that if I just find two sturdy shelves and but a board on top, I can put a mattress over that and make it a bed. That doesn't seem too safe to me. Is there any way to sort of do this without buying a bed, but still keeping it safe?