Your first job number is that which your job numbers will start from. You can choose between 1, 1000, or simply choose 'Custom' to define the number yourself.
In order to change this number, first, open your system settings. From here, open 'First job number' from the 'Jobs' section.
You will see a page with tick boxes, click on the number you would like your job numbers to start from. There will be help text telling you what number your job numbers will start from depending on how many you already have. To save your changes click 'Save'.