Hello BlueFlame!

If you seek to study the Highschool Chemistry here on Khan Academy as a 7th grader, I'd recommend that you begin with the Chemistry Library. 

The Chemistry Library basically contains a list of all topics found in Highschool for Chemistry. So you can basically begin with the Atoms, Compounds, and ions course - which is basically the basics of Highschool Chemistry. 

-Hope this was helpful!

~Galaxy

Thank you so much! I try it out right now!