Don’t put the dialogue tag and the dialogue itself in separate paragraphs.
Under normal circumstances, a comma, not a colon, is used to separate dialogue from its tag. It is sometimes acceptable to use a colon instead of a comma. Julia: “You’re such a dork. I’d rather date your brother.” (The Chicago Manual of Style says, “A colon may also be used instead of a comma to introduce a quotation, either where the syntax of the introduction requires it [emphasis added] or to more formally introduce the quotation.” So, y’know, don’t use a colon after said or any variation thereof.)
Capitalize the first word of each sentence!
Someone you know barges through your front door and says, “Whatever happens, whoever comes knocking, I’ve been here for at least an hour.”