Sam Vimes was about 16 when he enrolled in the Ankh Morpork City Watch about 30 years ago just prior to the events of "Night Watch" where he was part of the rebellion aginst mad Lord Winder and his Evil henchmen and was taught all he knows by on sergeant John Keel which is where his cynical outlook on life comes from. (Ironically Keel was in fact Sam himself flung back in time 3 decades.)
Sam was born in Cockbill street in the poorest area of the city below the Shades. It was so poor there wasn't much crime though Sam was part of a street gang with Lupine Wonse (later secretary to the Patrician Lord Vetinari) and at school was blackboard monitor for a whole term once.
During the first 25 years of his term in the watch Sam rose to command the Night Watch as it dwindled to a tiny stub as the power of the thieves guild grew. This insult to Sam's sense of justice and other events led to heavy drinking which probably came near to killing him. All that changed when Carrot Ironfounderson came to the city. This man who had been raised by dwarfs joined the Watch and set out to help the city (he's also the descendant of the last king of the city who Vimes' ancestor ironically done in). Around the same time a dragon assulted the city (no, not a swamp dragon, a bloody big real dragon) and the watch was instrumental in it's defeat. Later they took on new staff in the form of a werewolf, a dwarf and a troll and were instrumental in ending an attempt on the Patricians life. As a reward the watch was rapidly revived and became very important in the city. Vimes also (to his disgust) received a Knighthood.
to be concluded