I am planning to make my own BL novel

And send my manuscript to a publishing company who's asking for BL novel manuscript But the thing is I am Beginner at this. I've only written fanfics before but we won't know unless we try right? Anyway, My question is for people who likes to write books.

Do you prefer first person or third person?
Any tips for this starving and amateur writer?
How should I fix my grammar and writing?
Thank you for the kind people who answer.