Traditional Publishers tend to insist on very specific formatting and I'll put down here what the industry standard is.

However, always — always! — check the publishers guidelines before you submit and adjust accordingly if they specify what they want.

 

Paper Size

UK and Europe = A4
US = Letter

Margins

1 inch (2.5cm)
Top, bottom, left and right.

Typeface

Courier New 12pt.
Times New Roman is a proportional font
and doesn't space properly, but is acceptable
If you must use it, use 14pt.

Spacing

Double Spaced (for the story)
Single Spaced (Address details, Title etc)
Shoot for roughly 25 lines per page.

Paragraphs

Start every paragraph with a tab equal to 5 spaces.

Formatting

Any italics need to go. Anything that should be italics needs to be underlined.
If there is a section where you have a journal entry, or a letter, or something
along those lines, double tab (10 spaces) the section.
Do not use "Widows & Orphans" protection if your word processor has it.
If the chapter ends with a single word on a lone page… find something to cut, to fix it.
Only use one space after any punctuation mark. (Periods, Commas, Exclamation Marks, Question Marks, etc.)

Justification

ALWAYS justify left.
Anything else makes the text hard to read.

Headers / Footers

Every page has to be numbered.
Every page needs your name and contact number / email on it.

DO NOT STAPLE ANYTHING!