Next step would be to start writing it out. Personally, since my story is just a fanfic I don't really plan on completely making up a second draft, but if that is something you want to do, that's fine too. The important thing to know when you're writing is that things are not going to end up exactly how you wrote them in your book. Stuff always manages to find a way to deviate from the script a little, and that's alright. You've just gotta ask yourself if it adds anything to the story. If its completely random and contributes nothing, maybe scrap it. But it's your story, your rules. If you want to keep a scene because it's cute, go for it. Or maybe you can just write stuff as a little extra bonus if it doesn't fit with the rest of your idea.
It's VERY important to know that you are going to get tired of your story, or hit a block, and just not know what to write. That's okay, just step back and take a break until you feel better. Read some other stuff. Look at similar content to give yourself ideas. Its okay to need a break. It's better to sit back for a while than to hate your book for stressing you out.
Maybe you'll finish the story, and maybe you won't. But whatever you do, just think about what that story taught you, and think about if you had fun with it. You can use those lessons to make your next story even better.
Nobody's first one is perfect, and that's alright, haha. Just have fun with it, and don't let any pressure get to you.
That's all I can think of off the top of my head, I hope that helps! Apologies for any typos, I'm writing this at two am lol