There are a few ways to do this.
To reorder your tasks, edit the task you want to move and midway down the edit form is a "Move After" pick list. Select the task you want to move it after and it will reorder them accordingly. You may also reorder tasks from the Gantt by draging a task (or multiple tasks) by clicking (ctrl clicking) on the TASK NUMBER on the left, then drag above the TASK NAME in the list that you want to move them above. The icon will change to let you know when to drop.