What to do after I learn HTML , CSS , js?

Get a solid grounding and good experience in your existing knowledge, then learn a backend language (C#, VB, PHP, ...) and a server-based database system such as Sql Server or MySql.
This gives you much more flexibility in design compared to a relatively static browser based application you can build with HTML and Javascript: the "donkey work" is done at the server (backend), and the UI is handled in the client browser (frontend) to give power and responsiveness.

Without a solid grounding in both front- and back-end coding, you are limited to fairly simple projects.

And it sounds like your frontend knowledge is pretty patchy at the moment if you keep forgetting the syntax! Until you don't need to "lean on tutorials" you will spend too much time trying to get the code to run at all, instead of working out the "best way" to perform a task and implementing that.

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I keep forgetting what I have learn

In this site, you should find pretty much anything about learning those languages.
W3Schools Online Web Tutorials[^]

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but I don't understand what to do now

Practice, find some site subjects (any subject)and try to complete them, the goal is to practice.
By googleing, you will fi_nd tutorial site about making whole projects. you should find some video on youtube too.