How to insert process instance and processinstanceworkitem object in salesforce?

plz send .

my sample code:

public void ApprovalRequest()    {             

           ProcessInstance tempProcessInstance = new          ProcessInstance(Status='Pending',TargetObjectId='800U0000000HXxoIAG');        

                 insert tempProcessInstance ;                                            

  }          

error:

Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [Process Definition, Current Node]: [Process Definition, Current Node] .

process instance object cant have a Process Definition and Current Node.