If you are trying to access the Segments tool in Student Database, Applications or Campaign Manager you will need to be given permission from a User Admin from your Establishment.
Once you have been given permission, go into Student Database, Applications or Campaign Manager app and click on the Segments tab. You'll then be presented with a screen that shows all Segments. Click on the small Plus icon to create a new Segment.
On the New Segment screen, you can:
Put in the conditions you want to filter by,
Give the segment a name,
Add notes to explain what the segment is for
Chose to share it with other users, or keep it to yourself
Then click save and run, which will fetch all students that match the conditions you specified.
Next: How do I use Segments?