I use QGIS Bonn 3.2.0 So my question is how can i calculate Ha for specific fields i marked on picture. Is there any tool and if yes which and how to use it?
If you mean Hectare the function you are looking for is
$area returns the area as map units so in a projected coordinate system it would be meter. Thus you need to divide the result by 10,000.
Create a new field with type numeric/float and use the field calculator with the formula