The Fighter can join the majority of the Guilds, and is only somewhat hampered by race. The different Guilds really ARE different, their inhabitants as different as themselves. A fighter can join: The Black Bears, The Eldars, The Knights of Drin and The Scythe Clan. Any race of fighter can join the bears, any races besides orc can join the knights, any race but elves can join the scythe, and only elves and half-elves (helves) can join the Eldar.
If you are a new player, or value friendship and camaraderie over status and rank, choose the Bears. They are my favorite guild, as every single bear is a great, funny person. The main problem with the Bears is that anyone can buy from the Bear items bin. They can either snaek in or be invited in by bears. To join the bears either have a Bear sponsor you (ask them) or go 31n, 9w, 2n, 2w, 2s, and type join. You will then have to kill a grizzly bear.
If you are all about honor and valor and knights in shining armour, choose the Knights of Drin. They are my next favorite guild, and the guild members are almost as nice as the Bears, but there are more "bad apples". To join the knights all you do is enter their guildhall (12w, 7n, 1w, 10n) and type join. Note: There are Guards south and north of the guildhall who auto-attack orcs with some big damage attacks if you are a low level character. The high level knights can use guild points (got from killing monsters) to teleport back to the guild hall. There is no real drawback to the knights I know of.
If you are a loner and don't like newbies very much, don't like people very much, then join the Scythe. The Scythe are set up to be a "bad-ass" or something guild, but instead they just seem petty to me, getting off on pranks. To get there go 9w, 5s, walk. There are some nice people in the sycthe tho. The big drawback to the scythe is that they don't have an items bin like the other guilds, making it much harder for a new character to survive. To join the sycthe you need to kill something of a higher level than you. Lastly the Eldars. I know nothing about them besides that they are a guild.