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1. firstly, name the organisation the poster is released by in caps with big letters
2. start the matter straight away describing nutritious food and junk food [ good if decription maintained in the middle ]
note: generally a poster is prepared to call out people for some issues like Debate on the topic, a Presentation on it, Rally, Competition.
3.  then draw a cartoon or diagram suitable for it..
4. choose the one u like to call out the people .
5. write the venue, time, date and phone numbers to contact .
6. better if u mention a route map to reach the place.

Thank u and comment for changes to answer much better next time!! 
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 How must make the poster very nice, amazing, attractive, wonderful, colourful and interesting!!
   Here I can guide you;
On a big paper or board or something on which you will make your poster draw a very nice design and colour it to make it  very attractive.
 Then with deep red colour write 'NUTRITIOUS FOOD' on the top.
 Then draw the picture of an apple, draw its eyes, draw its two hands and colour the apple red, colour its eyes white and with black marker or sketch pen border its eyes, colour its hands with blue or green and then border the whole apple and beside the apple draw a pizza with eyes and hand and border it.
Next draw a sword in the apple's hand in such a way that the apple is cutting the pizza. (This means that a nutritious food is against a junk food and destroying it.)
Now write 'V' at the left side of apple and 'S' at the right side of the pizza.
 Then write 'JUNK FOOD' at the down of the poster with red colour.
 Mention necessary details.
I hope that this poster will be very nice and I also hope this answer might be helpful!
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