Someone has to train for sky diving. It's all about balancing in the air. You've got to learn how to jump with the clothing and still use the parachute without having any problems. Children below the age of 14 are restricted to go for sky jumping for the pressure up there is too much.
It is best to go to a USPA licensed dropzone. The age limit there is 18. You can do a 1st time jump either tandem or AFF. I recommend going tandem for your first jump and if you like it and want to learn how to skydive, take the AFF course. Have fun!!
Yes, you have to train because you need to learn stuff like how to set the parachute at the right time. And you have to be 18 years old to skydive commonly. But I suggest you don't skydive because if you make a mistake it leads to serious death. But there's your answer.