Unless your world is a lifeless wasteland there will be both plants and animals. Even on a spaceship there may be plants and/or animals for experiments or to help set up a new colony on a new planet. Not only this but plants an animals tend to form the basis of food, and materials.
Most people decide that the plants and animals of their world will be the same as those on Earth. This is alright, but it is possible to make new plants and animals for your own use. There will be different types of plants and animals on different planets because of differences in atmosphere, oceans, terrain, and sources of energy such as suns. It helps if you think of the possible ways plants and animals can adapt to survive in different environments. Plants on a planet with a sun that mainly gives off faint pale blue light isn’t going to rely as much on light. There might not even be that many plants on such a planet.
Another thing you need to think about is how people interact with the plants and animals. Which ones do they farm? How do they use them? Are there any belief systems that forbid killing certain animals or plants? Are any of the animals considered companion animals like cats or dogs? If there are going to be organisms other than your characters and their people in your world the people are bound to interact with the animal and plant life.