Make sure it's an really aquarium and not a terrarium. A terrarium is only built to be filled about halfway with water, which is not a good thing if you want to use it as an aquarium.
You can wash it out with water, or a little bit of diluted bleach if needed. If you use bleach, rinse REALLY well, and if it still smells even a little bleachy use some dechlorinator on it... decholorinator deactivates bleach.