Your first estimate number is the number that your estimates will start from. You can choose between 1, 1000, or just use your own custom number, to begin with. In order to set this up, open your system settings. From the 'Estimates' section, click 'First estimate number'.
From this page, you can simply click on the number you would like your estimates to start from. Click 'Save' once you've made your selection. If you click 'Custom' rather than one of the available numbers, you'll be provided with a text box in which you can enter your own number.