1) If you are writing in the past tense, stay in the past tense. Exceptions: a)when your characters are speaking to one another or b) when your characters are thinking and you want to emphasize what they are thinking.
2) There is a thing called Speech Attribution. What does that mean? It means that you use something like said, asked, interrogated, replied, inquired, etc. before/after the character speaks.
BAD EXAMPLE: I walked to the door, "What are you doing here?"
GOOD EXAMPLE: I walked to the door and asked, "What are you doing here?"
I walked to the door. "What are you doing here?" (note there is a period and NOT a comma after the word door)
EXCEPTION: If there is a conversation going on you can leave the quotation without speech attribution if a) it is standing alone, b) the sentence prior to it has ended, c) the sentence after it is new and capitalized.
3) “Convention normally insists that a new paragraph begins with each change of speaker.”
In other words, don't have two people speaking in the same paragraph. It gets confusing!
I hope this helps. Happy Writing =)