Block Sanding... and Filling?

I took a long block sander to the left fender of my '77 Midget this afternoon.  
What I thought was a straight panel turned out to have slightly high and 
slightly low places in it.  Yeah, I can tap down the high spots, but what goop 
should I use to fill in the low spots?  

I'm a newbie at this, but have seen the filler used on American Chopper and 
other great Discovery Channel shows.  Feel free to talk down to me, unless you 
know less than I do about this.  ;-)

 - Kendel