Just like research papers, it is important to give credit to where you find information to supplement your research in your academic posters. Those sources of information can be books, articles, newspapers, websites, personal interviews, etc.
If you are unaware of which citation style to use for your poster, consult your mentor.
There are a few ways you can do your citations!
You can have each citation at the bottom of the section for the information it's citing. For example, a picture's citation can be beneath the picture or at the bottom of the section that holds the picture
You can place all of your citations together in one section of your poster. It is common to have a "References" section of the poster for all citations to go. These can go along the bottom of the poster and in the columns with the rest of the text.
Have a look at the examples of posters in this guide to see how they did their citations!