I can't make a successful cobblestone generator. The lava always turns into obsidian. Please help me.
You're putting the lava and water blocks too close together. Try to keep them at least three blocks apart. It helps to put a temporary block between the two before you actually put the source blocks down, like so:
L . . X . . W
L=Lava, W=Water, X=whatever. Dig a trench 5-7 squares long, place the "whatever" block in the middle. Place the source blocks at the end of the trench, then remove the "whatever" block. That should get you started.