Firstly, look at the source and try to work out who, what when where and why it exists. For example, if it is a recruitment poster from World War II, consider whether it is a German poster or an alliance poster. This will inform the rest of your analysis as it will help you understand why it might have been published.
After you know who where and when, you can consider why and how. Aside from the context, you need to describe what is happening in the poster - this is the easy bit because its just a matter of picking out the interesting features.