Before you begin building your poster, ask yourself: What is the central message I'm hoping to convey with this poster? This could be why the research that you did was important, a particularly surprising or interesting result, or how you plan to expand on the research in the future. As you begin to design your poster, keep that message in mind; the choices you make about the poster's content and, to a lesser extent, design should be made to support that central message.
Research posters, by and large, follow a general set of guidelines without all being the same, much in the same way that research papers often follow a familiar structure:
It's a good idea to outline the key points you'll need to include while making a rough sketch of the poster.
Image from Indiana University's IT Training Tips blog
This will help you figure out how much information you'll be able to include, what kind of graphs or charts you may need to make, and how the content will look one the poster when finished. No need to get fancy, this is just a visual outline to get you thinking about the process.
It's not always easy to find examples of research posters, especially in a specific subject area. A good resource for tips, examples, and critiques of existing academic posters is the Better Posters blog run by Prof. Zen Faulkes of University of Texas Rio Grade Valley:
Another option could be to check repositories such as DigitalCommons, which is primarily used for articles but does also contains some research posters. There is a list of subject categories at the bottom of the page and, once in the chosen subject area, a search for "poster" should bring up relevant results. The Undergraduate Research Commons is a section of DigitalCommons specifically for undergraduate research, and posters there can be found in a similar manner.
In early 2019 Mark Morrison, at the time a doctoral student in psychology at Michigan State University, posted a video to YouTube detailing his concept of a "better" research poster. This model explicitly states the central message in extremely large letters at the center of the poster, with all other information pushed to a much smaller portion of the poster.
Morrison has posted PowerPoint templates for landscape and portrait-style posters to the Open Science Framework: