Basics
If you don't know One Piece then you should know it is centered around the idea of Pirates. The Pirates serve as the bad guys, and the Marines serve as the good guys. This doesn't mean that since you're a pirate, you are labeled as a villain. You may be a Pirate, and still protect civilians and such. Such as the protagonist, Monkey D. Luffy. He is a pirate, but he ins't a bad guy. He is a pirate for the adventure. This also applies for the Marines. Not every marine is considered a good guy. Such as in real life there are dirty cops. There are dirty marines, who act evil behind the scenes.
The Seas
The world of One Piece is mostly islands. The ocean that covers the world of One Piece is divided by 4 different seas. The North sea, the South sea, the West sea, and the East sea. With the Grand Line serving as the equator of the world. The Grand Line is a Ocean in itself. Past the Red Line the Grand Line turns into the "New World". Surrounding the Grand Line is something called the "Calm Belt" which is known to have the Largest Sea Kings* in the waters and have no winds.
Red Line
The Red line is a continent that circles around the entire world of One Piece. It moves along the Prime Meridian of the world of One Piece going North to South instead of East to West like an equator. As you can see in the image above, the Red Line splits the Grand Line into two haves.