All cars require services at different intervals depending on the make, model, age and mileage. Within the owners handbook there is usually a service schedule which will tell you what service schedule is recommended by the manufactures and also what items should be changed / checked at each service.
If you do not look after your vehicle and get it serviced when it should be then you run the risk of things going wrong with the vehicle which may have been preventable. If you are struggling to fork out the often £150 plus cost for a service each year then you may be able to take out a service plan which will allow you to spread the cost. Another way to save money is to do part of the service half way through the year. For example, if you drive a petrol car then the spark plugs are often changed every other service. Buying and replacing the spark plugs is a relatively simple job and means you would be able to reduce the service bill when it’s next due.
Always shop around from parts to get the best price, but you need to know exactly what it is you need to buy (and check fitment) and also decide if you want to use genuine or non-genuine parts.