This is probably the main mistake I've seen, and the most often done. Here is an example of a correct dialogue entry:
"Hello, how can I help you?" Will asked.
Katy shuffled her feet towards Will nervously. "I've come to collect a letter."
Will rummages through the untidy piles of books, packages and notes before finding a dirty, yellow envelope.
"Miss. Valentine?" Will questioned.
"Yes, that's me."
1. Dialogue should start a line down from the down from the rest of the text, simply for ease of reading in longer passages. When you stop dialogue, even for a sentence, you should then restart the dialogue again on another line.
2. Punctuation goes inside the quotation marks. No quotation means the text hasn't stopped. For example - "Miss Valentine"? is wrong. "Miss Valentine?" is right.
3. Last point - USE DIALOGUE SPARINGLY. It's much more interesting and easier to follow if you use dialogue at crucial points and describe the rest.
Capital letters are used at the start of a sentence, for people/places, and titles.
Monday, Peter, London..
1. They are only used for that, and after full stops. Not commas, or colons unless a place/person/title is following them.
2. Apostrophes - there is one basic rule for apostrophes, and that is "only use it when letters are missing."
You're - you Are
Aren't - are nOt
Homophones - Your and You're
You use you're when you could fit you are into a sentence.
You're good at art.
That is correct, as you could equally say "You are good at art."
Your cat is outside.
Correct again, as you CAN'T say "You are cat is outside."
These are only some basic mistakes I don't only see in para RP, but in everyday life with people, so don't take it as a rant at RPers, it's a simple guide to improve writing and makes posts more legible.