Leave a blank line between paragraphs. That's the only thing you need to know.
If you want to do additional formatting, be aware that beyond adding paragraph end and begin tags in the blank lines, everything else will be posted as you submit it. You may use HTML tags and characters, but please restrict yourself to text formatting (no links, scripts, images, etc.). Don't use <h1>, as that produces the large light-grey on black headline seen at the top of each page.
Special paragraph formatting may be achieved by including the paragraph end and begin tags yourself, suppressing those automatically added in the blank lines. For example:
Here is the end of an ordinary paragraph.results in:
</p>
<p style="text-align: center">
And here is the beginning of a paragraph that will be centered.
Here is the end of an ordinary paragraph.
And here is the beginning of a paragraph that will be centered.