There are 2 ways to obtain the training and hours needed to enter the commercial field, you can enlist in a branch of the military or you can go through a civilian training course.
The military route has its ups and downs. The military will train you and give you a pay check but in return you are required to serve an extended amount of time, assuming that you are accepted to the helicopter training program. If you are accepted and get to fly a helicopter you are guaranteed lots of flight hours but not always in the safest conditions. Once you leave the military and enter the civilian job force they still require civilian air time to become accustom to protocols and procedures that differ from the military training.
Attending a civilian training course allows you to pick and choose your licenses, you are allowed to learn at your own speed and do not graduate to the next level of training until an FAA certified testing instructor has passed you. In civilian training there are 5 different licenses that you can obtain, private, commercial, instrument, instructor, and instructor with instrument. If all five are achieved you hold what is called a certified flight instructor with instrument or CFII for short. The CFII license takes a minimum of 250 flight hours and roughly 80-85 ground school hours. The major draw back about civilian flight school is that you have to pay for your training. To obtain your CFII between 250-300 flight hours costs $75K-$80K.
Once a pilot has graduated flight school and holds a CFII at 300 flight hours they can begin instructing. Most all pilots that go the civilian training route will instruct to reach their 1000 hour mark. Once you have reached a 1000 hours you can then apply for a commercial position. The most popular commercial position is flying for the oil rigs in the Gulf of Mexico, where pilots can build their pilot in command hours rapidly. Other commercial positions available are sightseeing tours, heli-skiing, private transport, and news helicopters. After reaching the 2500 hour mark pilots can begin applying for medevac positions.
Although the path can be long and tedious to become a medevac pilot I do believe that the reward is worth the effort. The amount of experience and education gathered along the way will be immense. If one can afford the cost of private training I believe it is the route to go.