Start by thinking about who this presentation is for. So, in your case you will be presenting to your BIOL116 lab mates and some of the teaching assistants.
Next think about what exactly you want to tell them. For example, to understand your research, they likely need to know what your research question was, how you went about testing your hypothesis, and, most importantly, what were the conclusions that you drew, based on the research that you conducted. Some of the details of your work will be vital to the understanding of the work, while others maybe don't need to be highlighted quite so prominently on the poster.
Think about these things well ahead of time and make a list of all the things you want to say and put them in a list of priority. The top items should make it on the poster while the rest can be saved for your presentation. If there was one thing you wanted your audience to know (we often call this the 'take home message') what would that be?
Next take a step back, grab a pencil and start to sketch out a small poster to scale. What are the proportions you will be working with and what all do you need to include? Play around with the overall look placing content and diagrams/figures where you think it visually looks nice. A little bit of graphic design experience can be helpful here, but its not required. You can also do this by marking off an area on the floor or table base on the exact size of your poster, print off your content and/or diagrams and figures and see what needs to change.
Use a text hierarchy; meaning you have established a convention with the font sizes and styles that helps the audience easily recognize the order of importance of your information. Titles and subtitles should always be the largest, and visible from the furthest away (so that you can draw in potential readers to view your work). Other content can usually be a bit smaller (but still legible from at least 3-5 feet away).