Getting into the rhythm of a strumming pattern is something that challenges many beginners. The following video demonstrates a useful technique using an egg shaker to improve your strumming. A shaker that fits onto one of the fingers of your playing hand would also work.
I also like to use a finger shaker when playing with a group as it adds another dimension to the overall sound. Although don’t overdo this. One or two shakers in a group are enough. You don’t want to sound like a hail storm.