help plz confused with constructors

Hello I am new to java and I am confused on a project that I am working on. I have to design and implement a class called Sphere that contains instance data that represents the sphere’s diameter. I have to design the Sphere constructor to accept and initialize the diameter and include getter (getDiameter) and setter (setDiameter) methods for the diameter. I also have to include methods that calculate and return the volume (computeVolume) and surface area (computeArea) of the sphere. I also have to use the formula for volume and surface area where r represents the radius(of course) in a java format that I am also confused about. Also, I have to include a "toString" method returns a formatted one-line description of the sphere. if someone can help me by giving me some direction to approach this, I'll really appreciate it.