Joining an alliance is one of the best things to do once you have grown a bit in the game. Though the defualt alliance has protection, as explained in "Your Default Alliance: The UP" the Universal Protectorate eventually costs too much to stay in without losing the chance to get in the top 50.
To join an alliance, go to Alliance Listings and choose an alliance your are interested in. Once you are there, it will give you general information on the alliance and how they are doing. If the alliance is not full, there will appear at the bottom a button called Join Alliance. Click it and a request to join will be sent to the alliance leader. The leader of their alliance will either accept or decline you depending on the situation of the alliance and if he thinks you would be an asset to the alliance and meet their requirements.
Though requirements for alliances differ, below is a general outline of how some of the top line alliance conduct membership.
1. Ingame Activity-A player mustt be active in Alliance Wars to join most alliances. Without playing at least once a day, you run the risk of being a burden to your alliance rather than a helpful hand and will not be able to gain membership.
2. IRC Activity-A player must also, for many alliances, be active on IRC, an internet relay chat program, so that the alliance can keep in touch with you and make sure you are willing to put forth the effort to be a part of their alliance. Here is a link for IRC for Windows and Macintoch. Remember most alliances use GameSurge.net as their server and the main Alliance Wars channel is #aw.
3. Experience-Another critical factor is your experience level. Some alliances require you have at least a round of experience before joining but most alliances will let in new players to Alliance Wars.
4. Score-Score also determines your eligibility. If you are able to match their average member score, it will get you a more likely chance to join, though many alliances will let in players if they join in the middle of the round and can not mee the requirement because of that factor.
Remember to keep a positive attitude and good luck!