Code
trigger OpportunitySubmitForApproval on Opportunity (after update) {
 
    for (Integer i = 0; i < Trigger.new.size(); i++) {
 
        if (Trigger.old[i].Probability < 30 && Trigger.new[i].Probability >= 30) {
 
            // create the new approval request to submit
            Approval.ProcessSubmitRequest req = new Approval.ProcessSubmitRequest();
            req.setComments('Submitted for approval. Please approve.');
            req.setObjectId(Trigger.new[i].Id);
            // submit the approval request for processing
            Approval.ProcessResult result = Approval.process(req);
            // display if the reqeust was successful
            System.debug('Submitted for approval successfully: '+result.isSuccess());
 
        }
 
    }
 
}

Test Method

@isTest
private class TestOpportunitySubmitForApproval {
 
    static testMethod void testApprovalSuccess() {
 
        Opportunity opp = new Opportunity();
        opp.Name = 'Test Opp';
        opp.Amount = 100;
        opp.CloseDate = Date.today();
        opp.Probability = 10;
        opp.StageName = 'Prospecting';
        // insert the new opp
        insert opp;
        // change the probability of the opp so the trigger submits it for approval
    opp.Probability = 40;
    // update the opp which should submit it for approval
    update opp;
 
        // ensure that the opp was submitted for approval
        List<ProcessInstance> processInstances = [select Id, Status from ProcessInstance where TargetObjectId = :opp.id];
    System.assertEquals(processInstances.size(),1);
 
    }
 
}

Use the example for more ref

http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_process_example.htm