Navigate to your district's Homeroom login page with this link: https://yourschooldistrict.schooldata.net/homeroom, but replace 'yourschooldistrict' with your school district's name (i.e. Tahoma or MercerIsland). If you still are unable to access the login page, go to https://schooldata.net/homeroom and use the drop down to find your district.
From the Homeroom login screen, click "Activate Account" at bottom.
Homeroom will determine which SIS you are using and name the fields for you; in the example Skyward is the SIS.