For all the strum patterns and many of the exercises I am using a G chord on the guitar. A G chord can be played many different ways, but this is the way I play it (notice the first finger isn't actually playing a note):

Here's the chord diagram and tab to it:

 

Here's a second G chord you could play:

Here's the tab and chord diagram to this one:

 

Here's one last G chord you could play, it uses a barre at the 3rd fret.

Here's the tab and chord diagram to this one:

I doesn't really matter which G chord you use, any of them will sound just fine when played with the one I chose to use.