Help please

Im having shifting problems with the truck I just got it it seems to be shifting way rong it shift to real high gear while going slow and to low gear while going fast it seems to get stuck n low gear as well then shifts to high gear if u go through them like u would on a standerd but it is a {automatic} I think it might be out of a diffrent year of truck the trans would the computer me giving me this probvlem and who should I have look at