move 43- rh7+ wins for white instead of taking the pawn.
I would reccomend going out and buying a book on basic endgames. You need to know that r+k+p is also most likely drawn, with some exceptions and how to draw them.
Another alternative is to download this game, start from after you capture the knight and play against fritz or a similar program until you can win the position everytime.
A better answer to your question: For a K+R vs. K+R game, I don't think there's any way for you to force a win. You can only hope that your opponent blunders his rook away, for example, by allowing a skewer tactic, or by making a simple counting mistake.
Here, Black loses his rook via a skewering tactical blunder: