To learn to strum, it is good to understand and 'see' timing sense.

Most songs are of the 4/4 type, meaning there are 4 beats to one bar like this:-

||___x___x___x___x___||___x___x___x___x___||___x___x___x___x___

Each beat is known as a quarter beat, and each music note is known as a quarter note.

In the simplest strum, just use your pick and brush gently all down strokes for each beat, and give the 1st beat (in bold) a slightly stronger stroke.