This blog defines how to get selected feature from ArcMap.

IMxDocument pMxDoc = m_application.Document as IMxDocument;
IMap pMap = pMxDoc.FocusMap;
IEnumFeature pEnumFeat = pMap.FeatureSelection as IEnumFeature;
pEnumFeat.Reset();
IFeature pFeat = pEnumFeat.Next();
if (pFeat == null)
{
MessageBox.Show("No Feature Selected.", "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
System.Runtime.InteropServices.Marshal.ReleaseComObject(pEnumFeat);
return;
}
IFields pFields;
//Retrieving alla Fields value
 IEnumFeatureSetup enumFeatSetup = (IEnumFeatureSetup)pEnumFeat;
enumFeatSetup.AllFields = true;
 while (pFeat != null)
{
pFields = pFeat.Fields;
int indx = pFields.FindField("ID");
if (indx == -1)
{
MessageBox.Show("ID field not found in selected feature ", "Not Found", MessageBoxButtons.OK, MessageBoxIcon.Error);
pFeat = pEnumFeat.Next();
 continue;
}
object valu = pFeat.get_Value(indx);
 if (valu != null)
{
//Do something
}
pFeat = pEnumFeat.Next();
}
//Release Com object
System.Runtime.InteropServices.Marshal.ReleaseComObject(pEnumFeat);

}