This achievement is called "Streaker" and is awarded for talking to someone with no clothes on (that is, no armor or clothes equipped). You can nab this one as soon as you take control of your character in the Well of Souls. Simply unequip your shirt and pants and proceed along. After you meet your first gnome and finish talking to him the achievement should unlock.
Some TA members have pointed out that for some, they had to unequip everything (weapons, armor, and accessories) to get this achievement to pop so try this option if you are having difficulty yourself. Also feel free to check the comments for more info from these fine members.