Although some online publishers have their own guidelines, there is a standard format for manuscripts submitted to agents or publishers, which you should use unless specifically stated otherwise.
For the body of the manuscript --
Send loose pages; do not staple, paper clip, or bind. If sending an entire book manuscript, you may use rubber bands.
Margins: 1 inch, top, bottom, left, and right
Header: On every page but the first, includes title (possibly abbreviated if it is very long), your surname, and the page number
Font: Courier12 (this is safest, although some publishers don't mind Times New Roman 12)
Do not justify the right hand margin.
Double space
Indent at the beginning of each paragraph
For the body of the manuscript --
Send loose pages; do not staple, paper clip, or bind. If sending an entire book manuscript, you may use rubber bands.
Margins: 1 inch, top, bottom, left, and right
Header: On every page but the first, includes title (possibly abbreviated if it is very long), your surname, and the page number
Font: Courier12 (this is safest, although some publishers don't mind Times New Roman 12)
Do not justify the right hand margin.
Double space
Indent at the beginning of each paragraph