Soapstones are the most important aspect of the game. Use yours to get summoned into somebody else's world -- it's a no-risk opportunity to grab free souls and humanity, and it gives you a chance to see what someone else is doing to progress through the level/beat the boss. Alternatively if you HAVE humanity, you can summon someone else as a 'white phantom' and they'll help you through the level. 

If you find a glowing-orange soul sign, summon that person immediately! They're the 'sunbros,' and they're there for the sole purpose of making your life better. They'll guide you to all the hidden items, they'll beat up any invaders, they'll disarm any traps, they'll know the best ways to take on tough enemies -- they're awesome, and you want their help. It's usually worth risking an invasion if it means you can summon a helper.

[url=http://darksouls.wiki.fextralife.com/Guide+to+Co-operative+Play]here's a co-op guide[/url] with some general etiquette and some tips to help you find a good-guy phantom.