Step 1:

Go here
Step 2:

Click the sign in button at the top of the page and sign in.
Step 3:

Click the Join This Guild button.
Step 4:

Fill out the information on the page and then click the Submit button at the bottom
Step 5:

Thats it!  Just wait for the guild master to accept you.
Note:  If you are already in a guild, you must leave that one first.