How can write the Aggregate query and display the result of Aggregate query in Salesforce ?

For the Aggregate Query you have AggregateResult class in apex

Example

for(AggregateResult ar : [select count(Id), AccountId
                          from Contact 
                          where AccountId in :accset
                          group by AccountId
                          having count(Id)>2 ])
{
    contList.add(new Contact(Id = (ID)ar.get('AccountId')));
}
try{
    update contList;
}catch(DMLException exc){
    system.debug("Exception => " + exc.getMessage());
}