You will need to create a baseline. You will have to create your baseline > manage baseline

Go ahead and add a baseline. I set my color for the baseline to better view the coloumn. Then SAVE. 

Go back to Baseline in your top toolbar. 

The Baseline is a stamp in time for the time you add the baseline into your project. You are going to want to select the baseline you desire. 

You will see I have NA in my start date for task 26, this is because I created this task after creating the baseline selected.