HTML, CSS and JavaScript first. Then focus on how to apply that to server-side functionality.
There is no real "full stack". But once you learn the things listed above, you will find yourself working with a stack that works for you. LAMP, WAMP, MAMP, MEAN, MERN, MEVN; those are just sets (or 'stacks') of technology...
...but it's your skill set that's the true stack.