You need to be invited by the guild masters or the recruiting officer. As far as I can tell.

Most guilds will have websites where ypubsign up play with a few people if you gel, you get an invite, some guilds have cerimonies, others have quests for you to do to show your commitment.

Others just like to get to know you.

Invictusgc is one my and KNKID are putting together, were ex Socom clan leaders who led a clan to first.place in the European ladders but guilds is a new challenge for us.

Also, there are Guild Recruitment forums for the different platforms at the bottom of the Main Forums sections page.
Post a thread in there or look thru for guilds that look good to you.