Go Compare, money supermarket, compare the market, confused...
Generally a good idea to go direct to an insurer once you have identified which look cheap, as you may get it cheaper than the comparison site.
Sometimes more specialised brokers are worth looking at but if you are young with a normal car I found most were uninterested or uncompetitive.
There are various other tricks to bring down prices such as adding parents as named (not main) drivers, creative job titles, parking locations etc. some of which are detailed a bit more on money supermarket