The only way you can fly for a living is to obtain a commercial pilot license(CPL). Best compared to a commercial drivers license the CPL gives you the privileges of flying for compensation or hire. This covers everything from flight instruction and crop dusting to piloting large airliners.
Before you start training for the CPL you need to hold at least a private pilot license and an instrument rating is a good idea. At some point most also add a multi engine rating.
The CPL requires 250 hours of total flight time. These are broken down in cross country hours, pilot in command time etc. Most commercial pilots in the United States will have close to or over 250 hours when they finish.
Flying with a part 141 flight school the hour requirements are a little less. However you have to do your training from scratch (private pilot license) in a part 141 program. This way you can obtain the CPL at a minimum of 190 hours total time. It is also a requirement that you have completed the instrument rating, or currently enrolled in an instrument rating course.