When learning to strum, most of the movement should be coming from your wrist. Try not to move from the elbow unless you want to accent something or strike a chord a bit harder.
If your wrist is not loose, it can create a harsh tone for your strumming.
Developing a light movement is an important part of creating a nice sound. To develop this skill, work on one stroke at a time. Try and balance the volume, experiment with a light and fast idea, on the upstroke and then match the volume to the down stroke.
By varying where you strum this will alter the sound; close to the bridge creates a brighter sound while the further you move away from the bridge will create a darker sound. Find the sweet spot and use it as a tool for variation of your tone.
Having your pick slightly angled while you pick can help produce a sweet tone.
I can help you achieve your rhythm playing goals using the above and other ideas.
