Welcome to the site.
Are you writing a typical novel, or do you intend to convert it into a CYOA? Because that's going to affect how you do your plotting.
As far as grammar goes, there are tons of online resources for grammar review if you're concerned you may have forgotten some. But really, in fiction writing, you can get away with a lot of stuff you can't in formal writing. It just depends on your writing style and how you implement your breaks from grammar conventions. Fragments are a good example of this. The more you read and write the easier and more natural these deviations from formal grammar will come.
Many community members are happy to do proofreading for projects that are going towards the site, but I'm not sure you'll find a ton of offers for a work that isn't going to be posted here. I could be wrong, but I wouldn't hedge my bets.
If you look in the Help & Info section of the site, there are a number of articles on story-writing. Many mostly focus on CYOA story-writing, but the basic principles may be helpful.
But other than general information, you'll have to ask a specific question for help with the story and plot flow. Are you concerned with pacing, POV, character development, plot points, plot vs. story questions, foreshadowing, trope & cliche elements, etc.?
The basic advice I always give for any story is to use the active voice as much as possible and use strong verbs and nouns to decrease your reliance on adverbs and adjectives. The latter helps paint a clearer picture in less words, while the former keeps the reader engaged in the story.
But the best way to become a better writer is to write. And write the story you want, not what you think will be popular. Hope this helps, and once again, welcome.