Change Font Size and Line Spacing

Hello! My site is www.sanctuaryoaks.com. I would like to change the font size and line spacing throughout my site, but I don't know how. The font I want to use is Quicksand. I'd like 16pt for headings and 14pt for body. I'd like the line spacing to be single-spaced (it looks a little more than that now; I'd like it to be tighter).The color for the text is #555555. Thank you!