The maintainance booklet that comes with the handbook usually details the things that the service will look at. There are usually 2 or 3 types of service, eg a minor service and a major service, and other exceptional items are changed as service items when due. Its all done on mileage and age. eg cambelt is due for change at 70,000 miles or 5 years (varies from model to model). Anti freeze is usually every 2 years, seat belt pretensioners is every 7 years (but no-one bothers).
You will be suprised at how little is actually done in a standard service, and appalled at the cost. Watch out for 'standard' charges such as charging for refilling the washer bottle, when its already full. What type of service you need will depend on the previous service history. If you are not sure when things have been done before, it might be better to get a full service, and specify other things as well such as antifreeze, and even brake fluid.
But the maintainance booklet is a good place to start as this tells you the service interval and what is recommended.