Hello Rishika !

It is very good if you want to learn foreign language in 11th and 12th as it will help you in college also , because many colleges have foreign language as a subject .

Now coming to preparation you can start by you tube tutorials. Manage your time as you are in school , try to give 1 hour for the tutorials.

You tube tutorials teaches you from the beginning and will help you.

Also , there are many site which you can see ,but I will suggest you to go through you tube.

You can try this link : https://www.youtube.com/watch?v=4FB1nO6ckZI&list=PLKQEL9j11yiUQ5SWdkc6ZT6A1NRoaskst

Hope it helps !