Hi , Can you help me with writing test class for my trigger.

trigger orderproduct on Order__c (after insert,after update) {

    Set<Id> opportunityIds = new Set<Id>();

    Set<Id> orderIds = new Set<Id>();

    List<Order__c> orderList = new List<Order__c>();

    Map<Id,Id> mapof = new Map<Id,Id>();

    for(Order__c o : Trigger.new)

    {

        if(o.Opportunity__c  != NULL)

        {

            opportunityIds.add(o.Opportunity__c );

            orderIds.add(o.Id);

            mapof.put(o.Opportunity__c ,o.Id);

        }

    }

    if(opportunityIds.size() > 0)

    {

        list<OpportunityLineItem__c> oppprodList = new list<OpportunityLineItem__c>([Select id,name,Product2Id__r.name,ListPrice__c,OpportunityId__c,Sales_Price__c,Quantity__c from OpportunityLineItem__c WHERE OpportunityId__c=:opportunityIds]);

        // Loop through orders

        List<OrderItem__c> orderItemsForInsert = new List<OrderItem__c>();

        for(OpportunityLineItem__c pd : oppprodList)

        {

            OrderItem__c oi = new OrderItem__c();

            oi.OrderId__c = mapof.get(pd.OpportunityId__c);

            oi.Name = pd.Product2Id__r.name;

            oi.ListPrice__c = pd.ListPrice__c;

            oi.Unit_Price__c = pd.Sales_Price__c;

            oi.Quantity__c = pd.Quantity__c;

            orderItemsForInsert.add(oi);

        }

        system.debug('sucess');                              

        if(orderItemsForInsert.size() > 0)

        {

             system.debug('orderItemsForInsert +' +orderItemsForInsert);  

            insert orderItemsForInsert;

        }

    }

}