Make sure you punctuate dialogue correctly, particularly when using question marks or exclamation marks before the closing speech mark. So, for example:
“Who’s there?” He asked.
The sentence consists of the dialogue and the dialogue tag, so there shouldn’t be a capital letter in the middle of the sentence; even though a question mark is usually used at the end of a sentence, here it ends the sentence that is spoken, but not the sentence that the dialogue is a part of, so this should be:
“Who’s there?” he asked.
“I’d like a cup of coffee,” she said.
Note here that, although the sentence of dialogue has ended, the sentence itself hasn’t, so you don’t use a full stop at the end of dialogue, you use a comma.