The process of joining a guild in Oblivion depends on the guild.
Mages Guild and Fighters Guild - simply visit one of the guild halls in any of the major city holds. As long as you don't have a bounty on your head, they'll let you join.
Thieves Guild - You need to either have been told about the Thieves Guild by bribing a beggar (or just having a high personality and being naturally likeable) or by spending some time in a jail. If you take the jail route, someone will deliver you a note telling you how to join the guild.
Dark Brotherhood - Kill someone in the game. The message 'Your killing has been observed by forces unknown' may appear. If it doesn't, you'll just need to keep killing people.
If someone invites you to join their guild. Or create one of your one, but to create one, you must be a member. Otherwise, ask somoene if you can join their guild.
Some guilds have requirements to join.
the guilds > find a guild / join a guild
Talk to anyone in the Guild you want to talk to and you will be redirected to the leader.
A guild is a type of union. If you were a baker you would belong to the bakers union or a blacksmith would belong to the blacksmiths union. It is just that simple.
To join a guild click on the shield on the bottom left of the screen.
/join example-99999
Most guild you can join simply by clicking "Join Guild" on the left sidebar. If the guild is private, a member will need to invite you before you can join. If you go to the Guilds section on the Neoboards and create a new post saying that you are guildless/looking for a guild to join/etc., you will almost surely get many responses from people that would be happy for you to join their guild.
Simple. Just go to your guild and put the activitys on the guild front OR go to your pet page and do it on there heres an example http://www.neopets.com/~gween_jub_jub
The Trobbles in AQWorlds are in Marsh (go there by map or by typing /join marsh).
/join guru
type /join ledgermayne
/join marsh2
Type /join trainers
Type /join pirates
type /join shadowfall
type /join firestorm then your there