First of all, do not worry. The Treasury Department will mail most of the stimulus checks based upon the IRS schedule, but not all. If you filed your return late, then your check will be mailed out after the initial batch of checks are sent out. Additionally, if you filed your return close to the April 15, then it is likely that the IRS did not process your return before the deadline so you might be lumped in with the checks for late filers.
Everyone who will get a stimulus check will first get a letter from the IRS telling you to expect it. Your check is likely to be mailed out a few weeks after the letter, so if you have not yet gotten the notice then it could be even longer before you get the payment.
If you have not gotten your check yet, then the best thing to do is just wait for a few weeks. If you filed a return, then you have nothing to worry about. However, the IRS has setup a “Where's My Stimulus Payment” page on their website that will let you track the status of your payment.