Head to the official website of Tibia, click on the "Community" tab on the left. It will open. Click on "Guilds" and then choose the world that you want to join the guild with your character. You will need to apply for the guild first. Usually you need to talk with the Guild leaders before doing it.
Some guilds have different objectives different from the common focus on PvP combats (for example, some guilds want to be neutral groups, and others simply want to play RPG adventures), although almost every single one would be happy with powerful characters joining them.
Keep in mind that on some Open, Retro and Hardcore PvP servers, the powerful guilds will charge a "fee" if you want to play among them. Usually the biggest guilds on a server will work this way. Joining a guild by that way makes you much more protected, but also will mean that you will have duties and will need to be requested for wars and quest servers eventually.
Another commentary worth mentioning: Getting to know guilds is crucial if you want to transfer from a server to another. Even if you plan on not taking part on wars and being neutral, you certainly will need to have a talk with the dominant guild and explain your objectives on the server.