Quit Terminal - Do This first
Finder -> Go -> Go to folder -> ~/Library/Preferences # the tilda ~ is important
Drag com.apple.Terminal.plist to the trash
Finder -> Go -> Go to folder -> ~/Saved Application State/com.apple.Terminal.savedState
Drag data.data and windows.plist to the trash (and anything else you may find in that folder)
Finder -> Go -> Home
Command-Shift-Period # all the invisible files will magically appear in your Home folder
Rename or Drag to the trash the following files if and only if they exist
.bash_profile .bash_login .bashrc .profile
Again, ONLY if you do not want your shell customizatons
Command-Shift-Period # a 2nd time hide the hidden files again.
Re-Launch Terminal