There are two different r sounds: the single r and the double r, rr. I don't know if you know them, but there's a girls group named "Kara", the way "Kara" is spelled is the right way to pronounce the single r. Also, most japanese surnames spell the r the same way than in spanish, for instance "Kimura". Now, double r is more difficult. Put your tongue in your palate and start making noises as if you were a motorcycle, now try to say "perro" while doing that.

I try to explain it, but I don't know if can do it. In the letter 'l' the tip of the tongue (N.B. ONLY THE TIP) touches the palate. In the letter 'r' the tip of the tongue is up (like in the letter 'l') but doesn't touch the palate and it vibrates like the tip of the tongue of a snake. In double 'rr', the vibration is more long. Do You know the vibration of the tongue of the snake, don't you? Naturally the tongue of the snake vibrates out of its mouth. Insted you vibrate ur tongue inside ur mouth.