Many people playing survival mode will argue that one of the most fundamental things to create early on is a cobblestone generator. The generator is built off the simple premise that when flowing lava touches water it turns to cobblestone.
The first step in making a generator is to create two V-shaped trough. One trough will be one brick longer than another. There should be four spaces between the two troughs
In the longer trough dig two spaces straight down as seen in the picture.
Put a bucket of water in the back of the longer trough. If you have done it correctly the water should only extend one space out.
In the back of the other trough place a bucket of lava. Once placed the lava will continue to try to move forward but once it makes contact with the water it will turn to cobblestone. At this point you simple need to use your pick to harvest it.
This basic concept can be made to be more elaborate through the use of buttons, pistons, levers and red stone. It is possible to create an automatic cobblestone generator with enough patience, or if complex red stone isn’t your thing you can easily use a button and piston combination to make a semiautomatic generator.