Target macro melee class

Team - I am trying to get a better hand on how to target a mob while playing a melee class. ATM an DK. There is a reason while I play ranged as choosing a target is easier?! Now playing my DK it often informs that target is too far away, while I am trying to dps a mob right in front of me. Tanning around has only limited success.
So is there a target macro I can incorporate in my, DK.