Volume measurement

what is the best way to measure the exact volume of a specific area

A good flow would be to segment the area with the Segment Editor, then calculate the volume using the label map statistics in the Segment Statistics module.

Segment Editor: https://slicer.readthedocs.io/en/latest/user_guide/module_segmenteditor.html
Segment Statistics: https://slicer.readthedocs.io/en/latest/user_guide/module_segmentstatistics.html