autosomal dna comes from both parents, so your brother's test will match to people on both your mother and father's sides (unlike ydna which traces only the direct paternal line or mtdna which traces only the direct maternal). You and your brother will have similar results but the random way that autosomal dna is sliced up each generation means that you will probably get some overlap in matches but separate ones as well as he may get more or less than you do. There is a link to a very good beginner's guide to dna on one of the threads here...posted on August 23 on this board under title Beginner's Guide to Genetic Genealogy.
It takes some time to find the actual common ancestor in your matches and for many, you will never find it as it may be too far back in time, or you aren't matching to "fully" developed trees (or your tree isn't developed far enough to be able to see the matches). Good luck!