For this stuff, you don't need an amp, hey you can even learn without a guitar!

The key is rhythm!

If you can drum your fingers along with a song, you can learn to strum. If you don't yet have a guitar, all you need to do is get used to moving your strumming hand in time with the music. If you do have a guitar, mute the strings with your fret hand, and just stroke the strings with your pick hand.

Start out simple. Just one downstroke per beat, just as if you were conducting a band. Then add an upstroke once in a while to spice it up a bit. Leave the musical stuff aside for a while. Just work on making rhythmic, 'chuffing' sounds on the muted strings. Its fun! :D