Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Crash survivor (25 points): Complete Episode 1.
Locate the cockpit (25 points): Complete Episode 2.
Via Domus (50 points): Complete Episode 3.
Into the hatch (75 points): Complete Episode 4.
Behind the wall (100 points): Complete Episode 5.
Through the Flame (125 points): Complete Episode 6.
The way home (200 points): Finish the game.
815 (20 points): Discover all the memories in Flashback 1.
Sweet memory (20 points): Discover all the memories in Flashback 2.
Behind the glass (20 points): Discover all the memories in Flashback 3.
Identity crisis (30 points): Discover all the memories in Flashback 4.
Persephone (30 points): Discover all the memories in Flashback 5.
Whatever it takes (30 points): Discover all the memories in Flashback 6.

Additionally, there are seventeen secret achievements:

4 8 15 16 23 42 (15 points): Photograph numbers on sealed hatch.
A Turn of the Screw (10 points): Photograph A Turn of the Screw.
Beam of light (20 points): Photograph beam of light coming from the hatch.
Blackmailer (15 points): Convince Locke to help.
Boom boom (10 points): Open the secret door.
Button pusher (15 points): Enter the numbers.
Castaways' Treasures (10 points): Photograph, Kate's plane, Charlie's guitar, and Locke's wheelchair.
Desmond's Past (10 points): Photograph: Kelvin's jump suit Medical cabinet with vaccines Desmond's mural.
Dharma Ride (15 points): Photograph Dharma van.
Hero's hero (25 points): Save Jack.
Mapping the Island (10 points): Photograph the blast door map, the blast door map for the hidden entrance, and Sayid's cable map.
Pearl Station (20 points): Photograph the Pearl Station emblem.
Radzinsky (15 points): Photograph bloodstain on the Swan ceiling.
Save the castaways (15 points): Prevent the fuel leak from igniting.
Showdown (10 points): Successfully confront Beady Eyes.
Slave (15 points): Photograph skeleton in Black Rock.
Staff Station (20 points): Photograph the Staff Station emblem.