Q:
Guilds and What you do ?
A:
It depends on the guild you joined, what they expect from you and what you are willing to put in so most likely the answers will differ a lot. In some guilds you simply keep playing the game as you were with a guild chat tagged on. In other guilds they require you to be online at certain times and take part in certain events (like dungeons and pvp), contribute with gold towards guild goals and require you to be active on a voice server.
In this game you contribute to the guild simply by playing the game. By doing dynamic events, hearts, dungeons and pvp you earn influence for your guild that they can use to spend on guild upgrades (for example a guild bank). Depending on the size of the guild you are in (some are really really big) I would say in general a benefit is to play more often with the same people. Names and people become familiar and you get used to eachothers playstyles so dungeons are usually a bit easier when done with a guild group than with a PUG (specially when you have voice communication available).
When you are unsure what is expected from you, simply ask your guildleader. If something more is expected in that guild than you playing the game they can tell you.