If it is Outlook 2013 and if you have installed it using default settings, the shortcuts for all Office 2013 are located in C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Microsoft Office 2013.
Copy Outlook's shortcut from above mentioned folder, paste it to C:\ProgramData\Microsoft\Windows\Start Menu\Programs\StartUp to start Outlook automatically when any user of the PC logs in, or to C:\Users\Your_Username\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup if you want to start Outlook automatically only when you log in.
Remember that both ProgramData and Appdata are hidden folders. To see them in Explorer you need to enable Show hidden files and folders in Folder Options or Explorer Ribbon > View tab. Alternatively, typing %ProgramData% in Explorer address bar and hitting Enter opens ProgramData even when hidden files and folders are not shown, typing %AppData% does the same for your AppData\Roaming folder. Now you can click further down in folder hierarchy.
Kari