You can use vinegar and just put about a cup of it right in with the clothes.
Oxi-clean is also fantastic.
Have you checked the seal on the washer?? That's usually where mold hangs out. You need to pull it back a little and look, if you haven't. If you do find mold, clean it out and then keep a towel to dry the seal after every load. You should likely leave the door cracked open too.