List in making your own poster:

1) GIMP ? 

2) Background/ Texture

3) Brushes

4) Fonts

5) Color palettes

6) Character template

7) Render of characters/ Characters' pictures

8) PNG 

9) Quote ( not so important )

10 ) Title of the story/ graphic ( choose by yourself )

11 ) Tutorials in using gimp?

12 ) Tips in making poster (1)

13 ) Tutorials in making your own poster

All above are the simplest things that should have and know

So, do you have knowledge on or own everything like what have I listed?

If you already have, open your GIMP > File > New > Adjust the size > Okay > and start making your poster now...^^