Okay this is my second tutorial ever, so please excuse the noobieness.
Today i'm going to teach you how to make something like this
First make a new document ( I chose 350x125 )
Make a new layer by going Shit+Ctrl>N
Press D to reset your colours
Fill you background with black. It should now look like this
Get your brush tool and reset your brushes.
Scroll to the bottom of the brushes and you'll find this brush
Select the brush and change the color to white.
Aim at the top of the background and drag the brush from the left to the right so you get a result like this
Repeat below it untill you have a result like so
I hope you found this useful. Please comment