There are a ton of different schools of thought when it comes to play formatting. But there is no need to re-invent the wheel. For playwrights just starting out, we recommend downloading NYCPlaywrights’ Microsoft Word template, which has already taken care of the formatting for you, so you can get down to writing.
For more advanced playwrights, you’ll want to download specialized software to help streamline play script formatting. Celtx has a free version and considered pretty good. And if you have some money to spend, Final Draft is considered the gold standard.
(One caveat though: As you know if you’ve looked at submission opportunities, certain contests demand different playwriting formats. If an opportunity demands a unique format, it is annoying, but you should absolutely adopt your format to match those guidelines.)