I wrote on this subject on my own blog--but I'm going to get more specific here.
No matter how good you think your novel might be it needs to be looked at by an editor--not just a friend--but someone who know what good writing is.
I've read several self-published books by fairly new writers--some have been good, others not so much.
While reading the last of these, here are some things I noted.
First, point-of-view. It's fine to switch, but let us know it's happening, and please, stay in one character's head per scene.
What tense are you going to write in? Present or past? Decide and then stay there. (Yes, I know some writers use both, but they do it for a reason and know how to do it effectively.) Frankly, past tense is the easiest to handle.
Check carefully for repeated phrases and words. The book I'm reading now actually has the same phrase at the beginning and end of the same sentence.
Be careful of how you use an apostrophe--it is needed when you make a noun or pronoun possessive as in Bill's hair stood straight up, but you don't need it when you've made a word plural like all the birds are in the bush.
A pronoun should always refer back to the last person or thing mentioned.
In this particular book, there are numerous typos, page after page.
What's too bad is the plot, situation, and characters are good, but most readers will give up after a few pages. And do you think the person who attempts to read this will buy another by this author?
A good editor could have easily fixed all these problems. It actually makes me sad to have to write this, but hopefully it will help someone who is just starting out.