The are a lot of different ways to make a fire pit. From elaborate ones with mortar, bricks, and concrete. To simple loose fit ones made from bricks or rocks.
One of the easiest and safest ways is to use retaining wall blocks. These are interlocking blocks available in almost any color at your local home improvement store. You can play around with these to get any size or shape fire pit you desire.
As for safety, place your fire pit away from tree's, houses, sheds, ect.... Remember It may look a safe distance, But a heavy gust of wind easily carries sparks.