...few things...
...set a foreground color...
...decide whether to use the brush or pencil, set it to a size and tip style...
...draw you line in path mode, the icon next to shape mode in the top tool bar...
...shape mode asks for closed paths, so use path mode instead...
...then target a layer, go to the paths palette, ensure the path is selected in the paths palette, in the flyout menu of the paths palette you have stroke there...
...you need to set the size of the stroke by changing the brush or pencil size before stroking a path, and also selecting a foreground color you want to use...
...edit > stroke will stroke live selections...
Andrew