Set a baseline.... Tools > Tracking > Set baseline... for entire project.

This copies several reference points (including your start and finish dates) to other fields. Now add the fields "Baseline Start" and "Baseline Finish" to your table.

You might also choose to set Interim plans as the project moves along. This will copy the existing Start and Finish fields to Interim Start and Interim Finish fields, allowing you to compare the Actual finishes to the dates that were recalculated after there was a change (like maybe a scope change, or a task went horribly long), that makes your baseline plan no longer very useful. You can set multiple interim plans as your project is under way, each will copy start and finish fields to new fields (InterimStart1, InterimFinish1, InterimStart2, InterimFinish2, InterimStart3, etc).

I am concerned that you might be routinely entering dates into Start and Finish fields as you are entering tasks into your plan. In general, you should allow Project to calculate these based on your tasks duration or work, and that of predecessor tasks.