you can train against a computer program.
First, you can put the position you want to play on the board, and then play it against a strong program, while analysing it to see if you do the correct play.
If you don't understand why something is or isn't correct, put it in the form of a topic here on chess.com:)
hello, u say u don't now how to mate with a king`rook vs king.
There's a book by Capablanca where he teaches chess, and he teaches that particular ending (and a few others) because he says that before you master any phase of the game, the person needs to understand how pieces move and how to accomplish certain things at will in the board . One of those is the rook+king combination that never fails because there u mastered the combination of those 2 pieces.
Here's a video of it:
http://www.youtube.com/watch?v=WeMYBXEhCL0
2:08
The good thing is that capablanca really explains the moves not only "detects it".
hello, u say u don't now how to mate with a king`rook vs king.
No, I said I do know how to do it. 
Since it is an ongoing game I can't give you specific advice. What you should remember though is that you have an extra piece and therefore can attack a square more than your opponent can defend it. Focus down one pawn at a time and soon you will be able to storm any remaining pawns you have and force your opponent to exchange off his piece to give you an easy K X v K mate.
The game I mentioned is over now, so I thought I'd post it here. I won basically due to opponent errors, but I'm hoping I can find out more about what I should have been doing. See more in comments to game.
Basically, when you're up material in an endgame, you try to trade pieces, and not pawns. Then when you're left with one piece against zero, you try and snatch the enemy's pawns and promote yours.
Here is an illustration with your game :
Since learning is chess seems to primarily be based on experience (ie not many guiding principles that you can apply and calculation in each position seems important) one approach may be to take a position - say starting at move 36 and play it against a computer several times from the other side (you play white) and watch to see how it approaches the position. I've not done this but may be helpful.
yeah, endgames looks easy and cos you got lied by the little pieces left on the board :D and here is one video:
http://www.youtube.com/watch?v=HXJKn8fZnks
that guy is awesome!
I think I now need to do some work on endgames. Til now, endgames were kind of a rarity in my games--one player or the other had usually hung his queen before we even got to the endgame! But as I've been improving I've been finding myself stuck in (what are currently to me) difficult endgame situations more and more often. Getting some mastery of these situations should help me improve further.
What's "difficult" for me? Very basic stuff, apparently. Right now I'm in an endgame where we both have three pawns (the a, b and c pawns for each of us) plus a rook--and I'm up a knight. Though I'm up by a piece, I find I don't have the first clue what to do. I do not know how to turn this material advantage into a win--mostly, I think, because I don't really understand what I'm supposed to be doing with all these pawns in the first place.
So basically my question is--what should I read, or alternatively, what should I do, to get better at situations like this. I've taken a look at the tactics trainer/mentor functions here on chess.com. It appears I can get practice there on the basic endgames, but I am not sure whether knowing how to checkmate with a king and a rook (which I can do) helps me know what to do with an extra piece when there are three or four pawns and a rook on the board for each player. Are there exercises for this kind of thing?
(Also--there's a "trainer" in there for the bishop and knight vs lone king ending, but I cannot for the life of me figure out what I'm supposed to be learning from it. I can blunder my way through til I get the green "correct move" sign as many times in a row as is necessary, but there's no explanation for any particular move as to why it in particular is helpful. And there are like 27 moves to get through. I had similar experiences with some of the other endgame situations given in these trainers. Are there comments in them somewhere that I am somehow missing?)