to find diamond dig a stair case down to level 14(if u dont know what level your on press f3 and look at the y) and strip mine
You won't find many diamonds at that level anymore, all ores were lowered by 4 blocks in 1.8. I suggest going down to level 10 or 12 instead. (11 or 13 on the f3 menu, as the camera is about 1.5 blocks higher than the layer you are at.) Be sure to watch out for lava, as it generates at 10 and below.