Just some keywords to start googeling, search in this forum.
It is discussed all over the place.
CSS styles
Sigil - Epub-Editor,
Calibre - tweak CSS
Style:
p {margin-top: 0.75em}
e.g. will add a extra space atop of each paragraph.
Just to be complete:
To edit line-spacing you will have to alter
{line-height: 1.5em;}
This applies to the line spacing in a paragraph.
No,
JSWolf it is not necessary to point out that a line-height of 1.5em isn't recommended for e-ink e-readers