I've been a fan of 2 pairs ever since I did my DofE
Having never heard the things people say about cotton I wore one pair of regular cotton socks under a pair of cheap hiking socks and I was actually fine for almost all weathers.
For winter I either put another (!) pair of thick thermal socks over the top or just wore the themal ones with the cotton.
Saves your bulkier more expensive socks from getting sweaty as quickly and does wonders at stopping blisters.
Currently reconsidering my materials though, I may invest in some proper liner socks as others have recommended, as well as some better (merino or maybe bamboo) outers, just to allow my feet to breathe a little better.
as for tightness I say it depends on the boot but the most important thing for high-top boots is to ensure your ankle is well-supported, not only to prevent damage but also because rubbing around the top of the boot hurts like hell after a long day. and retie them at your first rest stop. I've never had too much of a problem with swelling, but actually I find my laces loosen and the padding gets compressed once you start walking so it's well worth readjusting as you go.