Before I start to learn web development, what basic things should I be aware of?
First of all you ask yourself in which field your interest in it.
Did you love Game Development, Software Development, Web Development.
If you love Web Development then you should start learning frontend techniques like HTML, CSS, and java script after that you should start learning backend programming languages like PHP, Asp.Net.
If you love Game Development then you should start learning UNITY gaming engine, Android, Swift.
If you want to build a software then you should learn c++, c#, Java.
Note: You may learn basic of programming first, data structures, algorithm, and design pattern.