How do I make(more than one) straight line?

[Ofnuts]

Your problem is that you have a very "anisotropic" brush. Dynamics are off so it won't rotate to follow your stroke(*). At the size you specify, when you stroke vertically the line is 20px wide but if you stroke horizontally, the line is as thick as the brush is thick: one pixel. And since you are zoomed out 4x, this is scaled to 1/4th of a pixel, so that 's a gray which is only 1/4th of the darkness of black.

Unless you know better, us the round brushes (2. hardness 100 in particular.)

(*) and given that particular brush, when it does (with Track direction for instance) you get the thin edge...