To get this achievement you need to find all events in the game.
There are five types which are:
Races (blue icons)
Road Rages (red icons)
Stunt Runs (green icons)
Marked Man (yellow icons)
Burning Route (brown icons)
The map at senocular.com is a well organized one that allows you to show views of individual groups of collectibles and discoverable items for this game. It also allows you to print maps. A majority of the events are located on the right side of body of water in the middle.
Here's the link: http://www.senocular.com/burnout/
You can check the number of events you've discovered on the pause menu simply by pressing
. The game notifies you when you've collected or found everything in an individual category like races or general events. As you progress through the game completing events and earning new licenses, you'll find a lot of new things. Once you've progressed through a reasonable number of races, you probably will have found a majority of the events in the game. You'll have few events to find, making this achievement much easier to get.