In Pokemon Diamond where is strength?
You can find HM Strength south of Soleacon Town at the top of the Lost Tower. One of 2 ladies at the top gifts you with this.
First if you mean strength you must beat the gym leader in pastoria city and she will give the HM strength. Then you teach to a Pokemon and you can use it. If you mean stealth rock you can teach it to a Pokemon if you have the TM for it or sometimes your Pokemon will learn it. I hope this is helpful