One of the most common ways to improve your CS:GO skills, is to change the crosshair in game. It can make you more accurate and improve your aim significantly.
There are two main ways to change the crosshair, via the in-game settings or console.
How to change your crosshair in game
Go to "Options", and then "Game Settings", under "HUD Options" should be different options for crosshairs.
How to change your crosshair via console
To enable console, go to "Options", then "Game Settings", and enable "Developer console".
Now in console, there are several commands to change your crosshair. The most common ones are:
cl_crosshairalpha - changes the transparency
cl_crosshaircolor - cycles between few colors
cl_crosshaircolor_b - sets the blue value in RGB scale
cl_crosshaircolor_r - sets the red value in RGB scale
cl_crosshaircolor_g - sets the green value in RGB scale
cl_crosshairdot - enable or disable the dot in the middle
cl_crosshairgap - sets the gap between the lines of the crosshair
cl_crosshairsize - sets the size of the crosshair
cl_crosshairstyle - toggles between several crosshair styles
We encourage you to try out different sizes and shapes to find the most suitable crosshair for you.
You can also use this handy tool to generate your own crosshair in CS:GO.