If you are taking out a loan, add on the repayments. Look at car magazines to see what is available for the price you want to pay, and to check the value of any car you want to trade in. Shop around and see what various dealers have on offer in your price range. Look for a reputable dealer - ask the advice of friends and look for a trade association sign. This should mean that the dealer follows the requirements of the Code of Practice for the Motor Industry, which could be helpful if you have a complaint.
Some dealers offer a fixed price no haggle deal, but in general, most car prices are negotiable.
It is up to you to negotiate the best deal. You may get a bigger discount if you pay by cash, but you may also be able to negotiate a discount if you arrange credit through the dealer. A dealer earns commission by arranging finance, so you may be able to bargain for part of the commission on the finance deal. Consider whether you are getting the best deal. If you are trading in your old car, make sure you know what it is worth.