Styles & Formatting is great in Writer but it is also a bit tricky until you understand some basics.
If you applied a line spacing from the Format > Paragraph and then try to change it in the Styles & Formatting window (F11), you will not see a change because the first formatting, a direct formatting over rules the Styles&Formatting settings.
I would try following: Select your text, right click and select Clear direct formatting. Under my above made assumption this will now let the line spacing set in Styles&Formatting window become visible. Next set your line spacing for your paragraph style to the value you want. All text with that paragraph style will adjust to the same line spacing.
As you are new I recommend you to set up your template or templates using the Styles&Formatting window. Direct formatting. Format menu or right click and change a format can still be used to make some at the spot changes when your styles don’t match your needs for a smaller part of your document.
Styles&Formatting might appear a bit difficult at the beginning but once you understand it the power of this feature is great.
I also recommend to have a look at the free guides which you can download from http://www.libreoffice.org/get-help/documentation/