Step 1: Lava Falls
Select your seed (preferably nyan) and start mining.You will probably find iron straight away if you do nyan.
Step 2: Buckets
So collect at least 3 iron and make a bucket after you cook it
Step 3: Collect That Good Stuff
Use bricks to jump up to the very top of the lava fall.Notice were the lava fall starts.Tap the start of the lava fall and ta da! You have lava! If you plant lava in water you get obsidian.Mine obsidian with a diamond pick axe