Where can I find Glowstone in MineCraft?

Mine your way out of the cave xD you'll find the rest of the Nether and there ya go

Go to the nether.

Try and dig your way into an open area. There you will see plenty of glowstone.

It depends. I'm not saying it in a mathematical way, but a x number of blocks in the nether are the equivalent of 8x blocks in the real world. It means that every time you walk 1 block on the nether, in the overworld it corresponds to 8 blocks (if you walk 2 blocks in the nether, in the overworld it corresponds to 16 and so on). So, if you walk a long way, you should spawn in another spot of the nether.
(Okay maybe it's a bit mathematical)