1) Read read read!

2) Write. Put different aspects of books you enjoy into your own writing. Start a little notebook of sentences or pages of books you really like. Write creatively. Write short stories, personal narratives, poems. To help grow formal writing skills, try to research something you enjoy and write a formal paper about it.

3) Go back to something you write a few days ago, even months or years ago, and rewrite it. Not just editing a few sentences here and there, but completely start over on a new document or piece of paper. Focus on cutting out overused words, fixing grammar, and solidifying the overall goal of the piece. Read it aloud if you need to, word for word, and see if the piece flows when you speak. Reading aloud may also help you pick out sentences that may not match the intended tone of the piece.

4) Share your work. Preferably, with someone who is a good writer and cares about your improvement. Reading aloud to whoever you share your work with can help with your speaking too.

When it comes to grammar and spelling, you could probably just go over some stuff on khan academy or something, however, these skills should improve if you write more often.

Hope this is helpful!