For Gimp;
Ctrl+a - Select All
Ctrl+d - Select None
There are many tools that you can use to create a mouth. I'm definitely no expert at this but you can use the line tool to 'sketch' the mouth and then use the stroke tool. If all else fails you can just use the brush tool.
This all depends, ofcourse, on the type of mouth you are creating. I suggest checking out the tutorials on making heads.