You may know I do a fair share of editing, and a lot of this editing involves work by new writers. Most of the problems I come across can be solved with one handy tip, a tip I'm going to share with you.
Get a good style guide and learn to use it. (The corollary tip is read Strunk and White's Element of Style.)
I am regularly amazed at how little writers know about basic grammar and punctuation. My job is made 3 times as hard by people who either don't know to put a space after a period or comma or are too lazy to do so. (I think some people are just lazy and figure they've paid for an editor to clean up everything. Let me tell you now, we aren't that well paid.)
So, if you are considering having someone edit your book-length work before sending it off to a publisher or agent, buy a style guide, like the Chicago Manual or an AP style book. Heck, even a 10th grade grammar book will work.
Here are a few things that I come across regularly:
Writing dialogue. This seems to confuse people most. This is the American standard for dialogue: "I am happy," said John. "How happy are you?" asked Mary.
The punctuation goes inside the quote. There is no space between the quote marks and the words.
Using attributes in dialogue. Dialogue should do all the heavy lifting, showing the reader action, feelings, etc. The attribute is mostly there to let the reader know who is talking. Said and/or asked are almost always good enough. I just read a draft of a book that kept using "revved" or "rapped" for said. I find myself paying too much attention to the attributes rather than the sentence.
Commas. People like commas a little too much. Best advice on when to use them: write the sentence with no punctuation and read it out loud. See when you naturally pause or stop. That's where you put the punctuation. (Another book recommendation: Eats, Shoots, and Leaves. You'll see that where a comma is placed changes the meaning of the sentence.)
Paragraphs. Please indent. I beg you to indent. And that means more than one space.
The word "then." I cannot believe how over used this word is. When it is used, it is usually because the writer gets stuck and doesn't know how to make the story go to the next thought. That's because the writer thinks the reader needs to know every tiny step and event. No, they don't.
Hm, I could keep going . . . Next time will be more common mistakes I come across.