You can link tasks one to another creating task dependecies:
To make a sub-task you can do one of those:
- Select a task in the left panel and then click Add>Add Sub-task on the top (or through the right click on any task)
- In the task creating/editing popup window click the "Parent Task" checkbox and then select the parent task from the dropdown.
- Drag one task into another - this is the fastest way
IMPORTANT! : The sub-task can NOT have the Start Date before the parent task one
The Timeline visualizes the task dependecies using connecting arrows. Moving the parent tasks will move all the dependent tasks as well, so re-scheduling is a breeze:
Read more about the Timeline 
