I'll second the recommendations for the
UO Stratics main site and
UOGuide. And FYI,
UOForums is UO's other big forum community. I'll also second the shard recommendations above. Be aware that many of those who want a crowded, lively experience have consolidated on Atlantic over the years. Elsewhere, you will often find yourself relying on your own good wits for entertainment. Both styles of play has fans.
UO Ultimate Codex is one of the biggest, most awesome informational sites produced in recent years.
UO Travelogues is probably one of the best (if unsung) websites and guides to UO ever composed. Like many old hands, the site's owner was a bit soured on UO by the time he wrote his final articles--a fact that has made me reluctant to keep pointing newbies toward his site. But, you can logically overlook the salt in his wounds based on the fact that it took him more years to get sick of UO than most MMOs last--and here you are on your very first day.
If you like all of those retro-style RPGs that you see on Steam and other pro-indie websites and platforms (I'm referring to everything from simple
roguelikes to big whopping adventures like
Avernum), then you should download the
Classic Client. If this is your style, then you have likely stumbled upon an adventure like no other, which will draw you in for years to come. (And if you like old games, UO also has a lot of
history behind it.)
If you like modern-style RPGs like
Linkrealms, Torchlight,
Path of Exile and Diablo, then you should download the
Enhanced Client. Graphically speaking, this newer client is far less impressive than other isometric games on the market. Its user interface, on the other hand (especially when coupled with the much-beloved
Pinco's Mod), is far more impressive than probably any RPG out there.
A cynic would say you're 10 years late to the party (UO is 15 years old, but subscribers peaked in 2003 at 250,000). Because of its age and sandbox game play, UO is capable of making an awful first impression. But people who stick around beyond that first impression have always, 100 percent of the time, had their minds blown by the depth of content (15 years worth) and ways to interact with that content. The biggest thing to keep in mind is that UO is a sandbox MMORPG. Unlike games like World of Warcraft, Lord of the Rings Online and Champions Online, UO does not direct you toward your next step. The devs have given us one of the biggest, most toy-filled sandboxes ever created. You're responsible for building your sand castle and everything in it. You're responsible for the richness of your play experience. Players who've become bored have tacked several years onto their subscriptions by continuing to redefine their "purpose" and goals within the game; by forging their own paths, and forging a path alongside their guildmates.
Edit: After you become established on a shard, ask here or elsewhere about any community sites your shard might have.