The Guild of Fighters is one of four guilds in Dungeon Lords. When you join it, you'll add the first tier fighter class to your character (unless that was the class you selected at character creation). All four guilds work about the same. For each one, you'll have to go on an initiation quest (#1), then you'll be given a second tier class quest (#3-4), then you'll be given a guild quest (#2), and then you'll be given a third tier class quest (#5-6). You can join two guilds in the game, but you can only select one third tier class for your character, so try to plan out your advancement before selecting any quests.
The Guild of Fighters has two guildmasters:
1 - Initiation Quest
Deliver the sword of striking to the Hall of Weaponry in Arindale.
This is a simple quest. You don't have to kill anybody, and the Hall of Weaponry is in plain sight. However, you won't get to keep the sword of striking at the end of the quest, so if it's a useful weapon to you, you might want to delay your trip to Arindale.
2 - Second Guild Quest
Find Sir Falen, a knight who was captured by the demigoth slaver Minas Taw, and rescue him from his predicament.
You'll find Sir Falen and Minas Taw together in a camp. After killing Minas Taw, talk to Sir Falen to complete the quest. He should give you a barbtongue runeblade, which is a nice (but basic) heavy weapon. If you're playing the Collector's Edition, you'll also receive 10,000 experience and a point of honor. Then when you return to the Guild of Fighters, you'll receive a heraldry bonus.
3 - Knight Class Quest (Class Information)
Find the Black Knight, Sir Tyran Bruvane, in Bolen Fields, and defeat him in a duel.
Bruvane tends to show up southeast of Lord Draemare's camp, near the statue there. He has a lot of hit points, but otherwise he's a normal melee fighter.
4 - Marauder Class Quest (Class Information)
Find the renegade marauder Guire Boarbone, who is ambushing travelers on the road between Fargrove and Arindale, and defeat him in battle.
Guire Boarbone looks like one of Barrowgrim's soldiers, complete with a grim shield. If you're running a version of the game prior to 1.3, then you'll have to be careful with this quest. If you run past Boarbone without seeing him, and then enter Fargrove, then Boarbone will disappear forever and you won't be able to complete the quest. So the best way to proceed is to accept the quest in Fargrove, and then head staight for the signpost west of town and wait there until Boarbone shows up.
5 - Lord Class Quest (Class Information)
Find General Crullis and defeat him and his troops in the Vale of Ruin.
Like all the fighter quests, this is a straightforward melee battle. When you return to the Guild of Fighters, you'll gain "the Defender" heraldry bonus.
6 - Deathlord Class Quest (Class Information)
Find General Trajek and defeat him and his army in the Vale of Ruin.
General Trajek will only fight with his sword, and his "army" will only consist of regular Barrowgrim soldiers, so he should be reasonably easy to kill. Once you've defeated Trajek and returned to the Guild of Fighters, you'll earn "the conqueror" heraldry bonus.