You've finished your first draft and are eager to query agents or editors. But don't! Resisting that urge and taking the time to properly edit your novel will increase its chances of success exponentially. That said, when editing your own work it can be really hard to look at your novel objectively. You worked hard for months, maybe even years, taking the hatchet to it can be a bit traumatic. But for the sake of the novel, it must be done. Take off the rose colored glasses and prepare to polish that baby up. The better the piece of work you submit is, the more professional you look.
Spelling & Grammar: Spellcheck is wonderful but it doesn't catch everything and it makes mistakes. Use it, but don't rely on it alone. Check thoroughly for grammar issues. To polish to a high sheen, read and be familiar with The Elements of Style. It's an industry standard for the most part. Root out too much passive voice and telling.
Arcs & Pacing: Check for plot holes, twist tie-ups, plot arc, and character arcs. Check for consistency and pacing (a read aloud round of editing makes this part easier). Don't be afraid to cut. Try to cut anything that doesn't accomplish something for your novel, meaning for the protagonist, antagonist, or the plot. If you find a scene that doesn't have a purpose, delete it. This is not to say that you can't set a scene, just don't go off on a wild tangent that isn't going to tie in later.
Format: Know the format publishers expect to see your work in. Usually that's 12 font, double spaced, 1 inch margins all around, with the title of the work & your name as a header and the page numbers as a footer. A title page with contact information can also be helpful.
How many rounds of editing you do before submitting is really up to you. A good rule of thumb is at least two rounds, more if you aren't spot on with your spelling and grammar to begin with.