It depends on what kind of fireplace you have. If it’s a masonry fireplace - those made of brick and block with clay chimney liners - you can install a wood burning or gas insert. Factory-built fireplaces – those composed of a metal fireplace with a metal chimney in a stud wall - are more limited in the number of available inserts. These can be replaced by a new high-efficiency fireplace.
This depends on many factors. Open fireplaces burn very hot and creosote build-up is rare. Open your damper and look up the chimney - you will know immediately. Slow-burning appliances are a different story. Check them often and clean them at least annually.