There are a lot of great resources talking about editing that you can look into. Stephen King's book On Writing is excellent. Anne Lamont's Bird by Bird as well.
Other suggestions:
Read outloud. If it's hard to read and you stumble over long passages or repeat the same words over and over, start rewording and chopping stuff away.
Consider all adverbs and whether they serve a purpose/move the sentence along. If you can get 'spoke loudly' from the fact there's an exclamation point at the end of the sentence then kill the adverb, etc. Not all adverbs are evil, but they tend to be a crutch for writing that doesn't need them.
Turn on your spellchecker! Like a bunch of folks here I edited for The Leading Edge, and it was STUNNING how many manuscripts we received that had basic words misspelled. Silly things like 'teh' instead of 'the' which a spellchecker would have caught. If you don't have a spellchecker there are plenty of them online, cut and paste your documents in and fix those basic words.
Get involved with a writer's group or hand the manuscript over to others to read. The caution here is that you can't give it to someone who will just tell you it's wonderful because they love you. You have to get objective readers that will tell you what they like and where they'd draw the red line and stop reading. Send to a variety of people of both genders. I've found that men and women read things very differently, and stuff that I would have cut based on input from my female readers are beloved sections by the male readers. Get lots of input and look for the middle ground so that you can continue to appeal to a wide audience.
Keep writing.
Keep writing some more.
And then keep writing some more.