Above you will see a simple map of the One Piece Universe. It is divided into five different oceans: North Blue, South Blue, West Blue, East Blue and the Grandline.
The single piece of land in the middle is a continuos strip of land that goes North and South. This land is called the Red Line, which typically houses the World Government buildings and the Nobles, while the rest of the populous live on islands scattered amongst the five oceans.
What separates the Grandline from the Four Blue Oceans is a thin strip of water called the Calm Belt. On this strip of water there is no current and no wind, causing ships to immediately halt in the middle of the ocean. Lurking under the water of the Calm Belt lay several monstrous fish called Sea Kings that can easily devour a ship in a single bite. In short, don't get caught on the Calm Belt.
To get onto the Grandline, you must ride a ship toward something called Reverse Mountain. It is one of the pieces of land that intersects all four oceans. There is a mountain that literally propels a ship up the mountain and down toward the start of the Grandline.
The Grandline itself is split into two parts: Paradise and New World. The end of the First half is where you encounter Marine HQ and have to either climb over the Red Line or dive under it going through Fishman Island. Then you will enter the final stretch of the Grandline, the New World. At the end of the New World it is said to house the location of the One Piece.