Do some research to determine whether reported instances of brutality or abuse have occurred in the jail you plan to sue. You can conduct a search online and in recent newspapers to find information to help decide if your case is similar to others that have preceded it.
Get the best attorney possible and make sure they are not from the locality of the jail you intend to sue. This will ensure that the attorney has no pre-existing affiliations with those in charge of the jail and will help you develop the best case.
Dedicate yourself to the cause and be prepared to stay the course. Filing this type of suit will be no picnic. Anytime you file a case against institutions deemed as "authorities," you can be in for a nasty fight. Be sure you are equipped to handle the stress and stand your ground.
Seek out others who have similar issues with the jail and consider filing a class action suit. If you have more people involved, you have more proof of criminal action on the part of the jail.
Know that what you are doing takes enormous courage and will have lasting influence on the treatment of others in jail. So few people who are abused and brutalized in jail ever do anything about it because they are scared. If you make the decision to stand up against the powers that be, you are helping the voiceless millions in the future.