Approval Process in Sales-force using Apex and Trigger

public void submitForApproval(Opportunity opp)
    {
        // Create an approval request for the Opportunity
        Approval.ProcessSubmitRequest req1 = new Approval.ProcessSubmitRequest();
        req1.setComments('Submitting request for approval automatically using Trigger');
        req1.setObjectId(opp.id);
        req1.setNextApproverIds(new Id[] {opp.Next_Approver__c});

        // Submit the approval request for the Opportunity
        Approval.ProcessResult result = Approval.process(req1);

    }

More info:

http://www.jitendrazaa.com/blog/salesforce/dynamic-approval-process-based-on-the-apex-and-trigger/