Go to your quest Journal. There are two tabs at the top, "Current" and "Complete". First see if you have a Main Scenario quest listed under "Current". If so, that's the next one you need to complete. If you don't have any under "Current", switch to the "Complete" tab, make sure the filter is set to "Main Scenario Quests" (which I think is the initial default anyway), and see which is the last one you completed.