Clan Tax (aka Guild Tax) is 0-5% of each Members Credits per day. Nothing else is "taxed". Depending on the Clan, these Credits are given out to Members for whatever reasons the Clan has: Help with Bids, reward advancement, just GP, or stolen by some Leadership Members. They should inform you about how they use Clan Taxes. (Except maybe the Stealing part; most Leaders won't admit the are just glorified Cargo Thieves, but it has happened several time before...)

Clan communication is by Clan Messages as a minimum. Many more organized Clans also use TeamSpeak or GSC, some form of IM service like Yahoo, and/or e-Mail. Again, depends on the Clan. And they should inform you about this.

Clan organization and goals are a blank slate: DarkOrbit has little information and almost no Rules for Clans: they appear to consider them a Player-Player sub-system. Many Clans have internal Rules or Codes, have a chain of command for Leadership, provide some kind of indoctrination/training, coordinate play times in some fashion, help their Members advance by helping/showing them how to take on various enemies, help them with Quests, fly together for protection and battles, etc. It all depends on the Clan Leadership team (which, sadly can be a dictatorship if the Leader decides that's what he/she wants). So the key here is a strong, honorable Leader and Leadership team.

Choose Well!