where do u get soft brush?

or if soft brush is a setting, how do u set it up?

cuz im trying to make a 300px softbrush to create a light source

you have a soft brush from start.
It is a round circle with a kind of outer glow around it.
If you hold your mouse over it the name of the brush will pop up.

Hard Brush = solid colour
Soft Brush = solid colour with a slightly faded outa part

The soft brush is good because it allows the brush to blend more with the image.

Under basic brushes

it's a round blur brush, you got it with your default brushes.

turn the hardness to 0%.