Guild fame is something that individuals acquire for the guild they are in, because of this you have to be in a guild to earn guild fame.
Guild fame is currently earnt through completing guild quests, there are various NPCs dotted throughout each land is at least one Guild Quest NPC, these NPCs hand out quests based on your current fame level.
The exception to this is that Apprentices have a different set of quests which can be accessed via the Crafters Corner building at 33,58 and taking the Portal to the Guilded Crafters area. The Apprentices can not easily do the normal guild quests as they can not kill monsters. For more information skip the following paragraphs and read the bottom of this page.
Hint: There is a Guilds of Hope building at 31, 58 which you can take quests from.
As you gain guild fame through quests you also unlock guild reward items, these reward items can be claimed from the Guild Rewarder NPC who is inside the same building as the Quest NPC.
Its worth noting that the NPCs only show the reward you earnt for a short while (or for a limited number of charges) after the charges or the time has expired the item is no longer available and the quest resets, allowing people to take the quest again.
These quests are repeatable, but once a quest has been completed you will not get guild fame from doing it until the quest resets, this is to stop people abusing the guild fame system.
Also to prevent one person from doing all of the fame quests, some of the armor quests can only be done by a player that will be able to use the reward. This is based on the Profession Armor ratings, so a Paladin can do Plate quests but not Cloth, Leather or Mail.
Apprentice Guild Fame
The apprentice class and subsequent evolutions are not able to fight so the guild quests normal players do can not be completed. The Crafters Guild area at 33, 58 inside the Crafters Corner therefore has a number of quests that the apprentice class can do to gain guild fame, yes it will likely be harder to gain than normal classes although to a dedicated crafter it may very well be quite easy.
The Apprentice class gets a different set of guild rewards, these are essentially the normal tools that you can craft except that they have higher bonuses and the level of the tool is dependant on your guild fame so the higher your guild fame the better tools you can gain.
Tip: As with any guild reward the tools are not sellable, can not be traded and do not dismantle.