[Newbie Question - Paint Straight?]

Hello,

 

I know this sounds kind of silly, but how in the heck do I paint in a straight line?

In Photoshop, you just hold down the Shift key and move the mouse to paint a straight line, but I cannot figure out how to do this?

 

Probably something extremely simple, but could not find anything in the help files.

 

Any help is appreciated.

 

Thanks.