can eny one give an idea how to start contest poster

First ideas that tend to help me are;
Research and research. And when you think you're done, do some more research.
Find something that hasn't be done or a gap in the market, or something that has the potential to be great. 
You would want to brainstorm as much as you can and always write down any ideas, no matter how silly they could be. 
After brainstorming and coming up with your ideas, make sure you know your target audience, your aim and your goal. 

Come up with rough sketches make sure you sketch a lot to see what you like. Each poster should have a call to action, some copy, and a hero shot. If you get confused make sure you use google to the best of your ability. Google is a life saver. 

From rough sketches, make sure you choose at least 3 different concepts to make better. Finalise and make them perfect. 

After your three finished posters, show friends and family and whatever is picked the most send it in. 

hope I help 

Thomas