I want to write a book, but my grammar is horrible?

I always have had a vast imagination, and have been making up stories that I would tell to my friends and family for awhile now. Recently a friend of my aunt told me I should write a book out of one particular story, she has published two books. I’m not completely sure about this, I love story making building a world and characters, but my writing skills are average at best. Is there any books or websites that can help me out with my looming problem?