Greetings Grumbler!
Welcome to UO and Atlantic!
There are many great guilds on Atlantic that can be very helpful! Sadly, Guild pride and rivalries have caused many to go a little to far on our forums... after the last couple of "looking for a guild" posts, many may be reluctant to start the rivalry war again - resulting in few responses to your post.
Please respond with a little more detail on your playstyle and I think that players will be better able to direct you. (Main question is on if you prefer PVP, PVM, or PVEverything!)
If you are very new, I would recommend a PVM guild until you get close to fully developed.
There are good PVM guilds by the hundreds on Atlantic, but the bigger ones have the advantage of more people on-line around the clock so you are able to participate in more things any time you are on.
Larger reputable PVM guilds include (In random order to not show any favoritism) but are certainly not limited to:
RK - Runic Knights
ODE - The Order of Drunken Elders
MSI - Mindless Self Indulgence
*E* - Evermore
With every larger guild, there will be a lot of great people, and some problem players... Take a little time to meet some players from each before you decide. I strongly recommend that you do a few trial hunts with the groups before you join to see if you get along well with them and also if they do things that interest you.
My own guild of ODE tries to be very supportive of newer players, but we also do not want to simply hand you the stuff to put you there now! We believe that if you work for something, you will appreciate it and enjoy it more! We would be more than happy to assist with starter equipment and information on how to gain gold, skills, and all the other items that make up today's UO. Most all of us generally play on a pure mage, or mage-tamer template.
If you would like to try a trial hunt with ODE, send me a PM here and I will be happy to introduce you to the group. We have guildmates and allies on at most any hour, but our main hunts are evenings US eastern time between 7pm and 11pm. We try to do something daily, although sometimes we just end up chatting in guild-speak as we train characters.
Again... Welcome to Atlantic and I look forward to meeting you in game!
Safe Journeys!
Tazar