Assuming you have some free trial time remaining (which you should) then you can create up to 8 characters per server. Once your free trial (30 days for new players, until 09/09 for legacy) expires then you will need to subscribe to the $14.99/month.

The problem you are currently facing is due to server restrictions that have been temporarily put in place. This is to counteract the high amount of traffic being seen on each server, and is related to the login restrictions currently in place. Square-Enix anticipates that the expansion to their data centers should be complete by early next week, after which we will hopefully see these restrictions lifted.

In the meantime, don't forget that once you reach level 10 in a class and complete your level 10 class quest, you can begin to join other guilds. You can have every class on one character :-)