You can get bargains if you leave it till the last minute but you will be lucky to get a flight to where you would like to go. If you aren't too bothered then that's fine.
However if you have decided on your destination and dates to take your holiday then you would be better booking as early as possible.
Airlines sell flights cheap early on to guarantee a part filled plane, as the plane gets fuller and the cost of running it is paid for they hike the price up! Only at the very last minute (a day to go before the flight) will they again knock the price down.
To find a cheap flight early you need to use websites that search a number of sites and not just one. Use one of the sites below or try all of them.