To create a new segment you have to follow steps below:
Click on "Lists"
Click on the list name of your choice
Click on "Segments
Click on "create new"
Choose a name for your segment
Select an operator match: "all" or "any" following what you need as a segmentation
To add "condition" for your segmentation, click on "+" icon at the right side
Select the "field" for your segmentation: E.g : email, first name,..etc
Select the "operator" and the "value"
you can many conditions as you like: campaign action or ecommerce action.
Once done click on "save changes"
To view the all subscribers of your segment click on "View subscribers matching your conditions"
You can export your segment by clicking on "Export segment"