As you were not too specific, it sounds like you have an existing masonry or built in fireplace. That locks you down to just an insert or special cover unless you want to rip everything out.
Per many, the greenest would be removing it.
- Gas is not really green and that would require some major changes including lining your chimney.
- For wood, make sure it doesn't pull the air from inside the house. That is, the insert / glass cover needs to fully close and seal well.
- This will require an air intake from the outside & this would be best handled by a mason / fireplace expert.
The biggest issue with these fireplaces getting the heat to spread to the house instead of going up the chimney. That would require a fan to help direct it out.
For more information:
Read "What is the greenest, best fireplace replacement?" a Q&A answered by Lee Hall.