Hi Monika, the first thing I would do is put it aside and work on something else for a week or two. That way, you can come back to it with fresh eyes. Have a read through, making notes about anything obvious (continuity errors, pacing, etc) and rewrite. If you belong to a writer's group then you may be able to source editing through them; don't forget alpha and beta readers, as well - the more polished the MS, the less work the editor will have to do.
Finally, once you think things are as good as you can make them, have a look for an editor - it's not cheap, but it makes all the difference. You'll want one that's familiar with your genre - a quick search should throw a few options up. Check into the testimonials, and maybe make use of a sample edit to see what difference it makes.
Hope this helps :) Going through rewrites and editing with my YA novel (more horro/action & adventure in my case) so I'm acutely aware of how many options there are. The above suggestions have always worked for me :)