The simplest method is:
- open the start menu (press the Windows key)
- in the search box, type "start"
- click on "Startup applications"
- on the new window, click on "Add"
- enter "Calibre" in the "Name" box
- enter "/usr/bin/calibre" (or whatever path is appropiate) in the "Command" box
You can find out the path to calibre by typing "which calibre" on the shell.