In Service Orders of Service Management

In the service orders,while inserting or modifying the record, if we dont enter “Service Order Type” field then i should not allow any user to move to Invoicing tab,the form should give message that you need to fill in “Service Order Type”.Can anyone help me in getting done.Do I need to write code then in which trigger and what code. Please help.

Your question actually is about mandatory field - this has been discussed here extensively already.

Please search the forum with keyword “Mandatory” or “Default” and you will get a lot of forum threads to help you solve your particular problem.