Ok here r some details, i have been mining for like 3 days now(strip mining)all i found was gold and iron and redstone but no diamond, anyway i kept searching but then i found something like coal ores but i can't mine it i even have a golden pickaxe(you won't believe this but i got 46 gold ores from this mining mission).
also another question i've been lookin up question in this site to look for diamonds but they said that to go y=12 or something, what the hell is that?
The coal like ore is probaly bedrock, un breakable unless in creative mode. Y is the level your on, y= 12 and under is the best spot to find diamonds.
A golden pickaxe isn't strong, use an iron or gold pickaxe. Also on your map, it will have coordinates, look at the number next to the Y, and it will decrease as you go underground. When it gets to 12 (I prefer to mine at 11) there's a higher chance of finding diamonds. You need an iron or diamond pickaxe to mine diamond. Use the gold you find to make golden food. In real life gold is a soft, bendable, edible metal that is not strong, so I wouldn't recommend making tools from gold.
Basically Diamonds appear between 1 and 16 blocks up from the bottom of the world. That's what Y level means. Y=0 equals the layer of unbreakable bedrock at the bottom of the world. Y=12 is twelve blocks above bedrock, on on average about 40~60 blocks down from the surface. Unfortunately there's currently no way to actually see these number in any edition of Minecraft except for the PC version.
Gold pickaxes are statistically the worst pickaxes in the game. It can only mine the same things as a stone pickax (ie not Diamonds, Redstone, Gold, etc) and has the lowest durability, breaking after only 23 uses. They do enchant very well and are faster than iron when mining what they can mine; but this is probably not worth the downsides. Iron picks are always a better choice until you have diamond picks.
The coal-like substance you found is probably bedrock, it is unbreakable in survival mode, and dangerous to break in creative mode (as it creates a whole out of the world, fatal if you fall in.)