There are two ways to become part of a guild: creating a guild, or joining one.
(Players can only be in one guild at a time.)
Creating a Guild
Creating a guild costs 25,000 gold and can be done via the guild section. The new guild-master then sets the requirements to join, including a level range for prospective players.
However, a guild with one player isn't particularly attractive to join, and most players would prefer to search for better-established guilds with more players and larger payoffs.
In order to effectively promote a new guild (or an existing one), the guild-master can advertise in Gems of War's global chat channels, or on the community web forum in the Guild sections.
Current members can also send invites to other players, although they will have to not be in a guild in order to accept.
Joining a Guild
Joining a guild is far easier than creating one yourself. The main ways to join are by searching for an open guild in the guild section, or by responding to advertisements in global chat or on the web forum.
Existing guilds can vary in style, and finding one that suits you may require a degree of research. They range from quite casual all the way up to the most hardcore, which tend to have stricter requirements and weekly contribution & activity quotas.
Creating a guild enables you to make all the rules and set the requirements, style of the guild shield, and the name of the guild. However, it is a slow process and not guaranteed to succeed. It also costs 25,000 Gold.
Joining an existing guild immediately gives access to the established community within and the bonuses the guild has access to. The only risks are that you aren't a good fit for the guild, and that it may some moving around to find a guild you like - but this can be mitigated by doing research in advance or simply joining a known, successful guild.