Some people have the notion of posters as something very over-elaborated, but since posters are one of the most used means of advertising something, opting for minimalism helps avoiding distractions and makes the expression less is more be actually true. When creating posters, try to keep the number of geometric shapes and text on the lower end. Additionally, Freepik has lots of posters in many different styles, including minimalist ones, so you can check them out and find inspiration.