As far as grammar, reading will teach you grammar. Grammarly and Google can fill in the rest.
Build-in brakes during the writing session, and this is the time to check your electronic devices.
To support your work, subscribe to a free application that will monitor your progress in terms of spelling, grammar, and style. I suggest Grammarly. It's free and easy to use.
Writing is an art and it requires work and revision. Create an outline and treat your first draft as a way to "get your ideas down." Then your next drafts you will edit and revise your work.
The secret is to read and write every day.
It's great that you are already writing. The best way to make your writing better is to get feedback and improve. You can join a small community or can network with some writers on social media and connect to get feedback on your writing. But before that, I will suggest you to go through it yourself and find gaps and fill them, reread and edit where you think some improvement is needed, repeat this multiple times. You can also join some online sessions where you can share your work and welcome criticism.
Some tips for Young Authors as you are:
1. Don't Stop, continue to write and write.
2. Read voraciously. Find the style of writing your like, and then explore what is your style of writing.
3. Write about things you know and things you love. This will pour in a special touch in your writing.
4. Also Research areas you are writing on, its fun.
5. You can start with writing short stories. You can also write for some publications or guest blogs for a start.
6. The most important thing is to love writing and to take criticism like a champ and improve.
Again Read, Write, Relax.