Before you send your manuscript to a professional editor/proofreader it is always a good idea to complete several rounds of edits on your own. I believe that a three-pass system works best, and here is how I approach each.
1) Structural Edit
This pass is mostly about checking for plot holes/inconsistencies and reviewing the pacing. Look at language, too, but this first edit is more about ensuring cohesion.
2) Copy Edit
By now the larger issues should be resolved, so during this stage you should pay attention to the prose, tweaking for readability and comprehension, removing repeated words, and fixing grammar mistakes.
The final pass is a standard proofread to check for any remaining typos. At this stage you should also ensure that the formatting conforms to the publisher’s preferences. (Good practice is to set up correct formatting right from the first draft, if you know that far in advance to whom you’ll send the book when it’s finished.)