Editing and proofing are both skills that can be acquired with time, hard work, and a focus on serious learning....and some people will still be much better at it than others.
If you don't have a lot of experience with either and don't have the money to hire qualified professional help (the qualified is important!), then here are some things to do and tricks that may help the proofreading process. (Editing for story, do ask some people to be betas for you.) Start with number 1. You can do all the other points in any order. Sometimes tackling it in different ways at different times is helpful.
1. STUDY books on editing and grammar. Those that offer checklists of things to watch for may prove helpful.
2. Look for lists of commonly overused words (my personal favorites: just, really, very, and any word I happen to like a lot for that particular book
). With those lists as reference, put your manuscript in Word and do a global search to see how many times you've used each word. You'll be amazed at how often those suckers creep in. Edit out as appropriate.
3. Of course, run Spell Check and grammar checker. Those won't always be right, but work at understanding why things are being highlighted, especially for grammar.
4. Print it out, double spaced, and edit by hand, starting both at the beginning, and, for more detailed looks at punctuation, etc. from the end and working forward (If you're starting from the end, you can't get caught up in the flow and so can see what's actually on the page more clearly.) Also, when starting at the back, you'll be looking at each sentence. Here's where the books on grammar and sentence structure and word use are important....look at the sentence, not just the message it adds to your story, but the way the words and punctuation actually fit together. Ask yourself if that sentence can be shorter, clearer, more powerful. If it's just filler, take it out.
5. Format it as an ebook and send it to yourself to read on your Kindle or your phone, NOT on your computer. Take time to format it properly first (Scrivener makes that easy), because you're more likely to spot problems if you're reading a "book" as a reader and not a writer looking at a manuscript.
6. Set it aside and let it rot for a while. Gremlins get in and ruin your brilliant work. It's easier to spot their damage if you've got some distance from the creative effort.
7. Set it aside again....then do it all a few more times. Accept the fact that even if you know what you're doing, it's going to be a substantial amount of work and a lot of time, and the less skilled you are, the more time and effort will be required.
Good luck. It can be a steep learning curve, but it's a skill you need to acquire anyway even if you eventually can hire the best professionals, so you might as well start learning now.