You want to use the GetSchemaForGame API call.
http://api.steampowered.com/ISteamUserStats/GetSchemaForGame/v0002/?key=YOURKEY&appid=APPID&l=english&format=json
YOURKEY
is the API key you have gotten from Valve. APPID
is the application ID of the game you are looking for.
The API has the following format (taken from the above Wiki link)
game
gameName (string)
Steam internal (non-localized) name of game.
gameVersion (int)
Steam release version number currently live on Steam.
availableGameStats
achievements (Optional) (array)
name (string)
API Name of achievement.
defaultvalue (int)
Always 0 (player's default state is unachieved).
displayName (string)
Display title string of achievement.
hidden (int)
If achievement is hidden to the user before earning achievement, value is 1. 0 if public.
description (string)
Display description string of achievement.
icon (string)
Absolute URL of earned achievement icon art.
icongray (string)
Absolute URL of un-earned achievement icon art.
stats (Optional) (array)
name (string)
API name of stat.
defaultvalue (int)
Default value of stat.
displayName (string)
Developer provided name of string.
With a data result (from TF2, appid 440) looking like this:
"achievements": [
{
"name": "TF_PLAY_GAME_EVERYCLASS",
"defaultvalue": 0,
"displayName": "Head of the Class",
"hidden": 0,
"description": "Play a complete round with every class.",
"icon": "http://media.steampowered.com/steamcommunity/public/images/apps/440/tf_play_game_everyclass.jpg",
"icongray": "http://media.steampowered.com/steamcommunity/public/images/apps/440/tf_play_game_everyclass_bw.jpg"
},