What and where do I need to put the OreGen in this pile of code...

 

http://pastebin.com/LJ2pJMzE

You probably want to create a new class. Putting everything into the main file is a terrible idea. Have the newly created class implement IWorldGenerator. You will need to implement the method generate();

The generate method gives you the chunk and world that it should generate in. Now, adding any code into the generate method will run in all dimensions. If you want to only generate in the overworld, you make sure that the world's ID is 0. You can get the current world's ID with "world.provider.dimensionId". -1 is the nether and 1 is the end.

 

Next you want to make a for loop that loops through the intergers 0 to any number of your choosing. The higher the number, the more common your ore will spawn.Then add this inside of the for loop:

int firstBlockXCoord = i + random.nextInt(16);
int firstBlockYCoord = random.nextInt(maxHeight);
int firstBlockZCoord = j + random.nextInt(16);

This will choose a random block to start generating the vein.

Finally, you will want to create a new WorldGenMinable and give it your block and max vein size and then call the generate method in the WorldGenMinable.

 

Now, to make forge use this generator, you will want to add this into your init() method.

GameRegistry.registerWorldGenerator(new OreGenerator(), generationWeight);

 

NOTE: I wrote this using 1.7.10, so things might be different but not much should have changed.

There's probably a YouTube tutorial on ore gen out there. Search the web, specifying your minecraft version.