For Black and White one 1, you simply go to the house left on the Pokemon Center. Talk to the girl in the house. She will ask if you can show her a move from the TM HM list. Dont worry, she will only ask for a move that you own. Once you do that, she will give you a Heart Scale.
After reciving the Heart Scale, go to the house to the right of the Pokemon Center. Talk to the lady in the top left corner of the house. Talk to her. She will ask you if you want a Pokemon to remember a move. Say yes and click Samurott. Megahorn should be the first move there. Click it and then forget one of Samurott moves.
Easy as that!! Hope I helped!! :)