You don’t need to learn to much about it. I would suggest read a few presentations on what a relational database is and how it’s built. The relational part of it is what makes it so widely used. There’s also NoSQL (non-relational) databases like MongoDB that is getting popular with Node.
Should I learn Java or Python first?
If you’re just interested in programming and want to dip your feet in without going all the way, learn Python for its easier to learn syntax. If you plan to pursue computer science/engineering, I would recommend Java first because it helps you understand the inner workings of programming as well.
No, not at all. PHP and JS are both separate things. So you can go ahead and learn php without knowing JS at all. Although i must say that you will definitely need to know JS, when you will become an intermediate developer, to create client side effects like client side validations, ajax, etc.
Should I learn SQL or PHP first?
Start with PHP to understand concepts like variables, functions etc then learn SQL.