The best designs come from spending time before you start.
Steps you might try:
Brainstorm several ideas.
Sketch out those ideas.
Get some feedback from someone you trust.
Select one or two that seem to be the most successful or most meaningful to you.
Choose the tools you will use to create this piece.
Create your poster and make changes or new versions as needed.
This process is often iterative so be ready to go back to the drawing board.
Digital tools are a great way to create your own poster!
Here are some tools you can try:
Creating a 2 dimensional poster using physical materials can be as easy or as challenging as you want!
Try the following methods to create your own:
Draw (pens, pencils)
Collage (paper, found images, digitally printed elements)
Stamp/Imprint (with rubber stamps, potatoes, linocuts)
Or a combination.