If you have already created a task and want to link it to a Contact, the way to do this is as follows,

Step 1

Click on 'Tasks', which can be found on the sidebar menu to the left of your screen. You will see your list of Tasks here.

 Step 2

Click on 'Edit this Task' button (pencil icon), then all you have to do is to search for the Contact Name within the Task name window by using the @ symbol after the name of your task, and then click to select your contact:

Step 3 

All you have to do now is press 'Apply Changes'

The same process of searching for a contact by using the @ symbol in the Task name applies when you are creating a new Task.