Tibia is strongly based on the community and one's relations with others. Guilds strengthens one's relations with others in the world.
The guilds could be organized guilds made to bring wealth to their members, to help other players or just as a family or group of very good friends.
To join a guild, first go to the guilds page. Then, scroll down to and select the world in which the invited character resides. Afterwards, find the guild and select the view button on the right and enter the guilds page; once in the guild page, select the 'Join Guild' button. As a side note, you must be logged into your account to join the guild.
To create a guild you need a Premium Account and also 4 vice-leaders with Premium Account too.
Note that only one character per Premium Account can be a guild leader or vice-leader.
Side note: The guilds leader can decide to rent a guildhall. This is a great opportunity to rent a room, even if you are a free account player.