I think you meant concept art, not conceptual art (two very different things).
To design unique and interesting environments, you have to analyze the world we live in. Learn about the different architectural style of different time periods–from caveman all the way to today’s ultra high-tech buildings. Don’t just look at pictures–you have to learn about the functionality too. For example, the way a castle is designed has very specific functions for defense against invading enemies. When analyzing different public spaces, you have to think about why a hospital is designed very differently from a restaurant or an office. They all serve different purposes, and you must be able to design for purpose, not just for looks. From the air circulation, emergency exits, doors, hallways, lobby, lighting, interior decorating–they all need to match the purpose of the environment.
For natural settings, you have to study geography and look at many different types of landscapes and understand how they were formed. Different geological conditions will form different environments, depending on how much rain there is, how hot or cold it gets, what kind of natural disasters takes place there, what the altitude is, what historical natural events has taken place there, and so on. A rain forest exists because it is in a region that has very different conditions compared to those of a tundra, or swamp, or mountain, or cave, or seaside cliff, etc. You need to understand these things, and your knowledge will inform your designs.