William The Bloody was brought to eternal life by Angel (that makes him his sire) and was taken care of by Drusilla, who he fell in love with.
William gets an hobby (torturing people with railroad spikes) and earns his nickname, Spike.

Then Angel disapears due to restoration of his soul; so Spike and Dru are forced to go solo. They move to Prague, stay there for a while, but almost get killed by an angry idiot mob. That makes Dru weak so they move again, this time to Sunnydale, hoping to find Drusilla's sire, Angel. Unfortunatly, they find the Slayer too.
Spike killed the Annointed One (that made him the boss) because he wanted "a little less ritual and and a little more fun around here".
At the same time, he meets Buffy.

He tried to kill her but he failed. Then Spike finds a cure for Drusilla. He has to get the power from her sire , Angel (still good at the time) and move it to Dru's body. That would (of course) have killed Angel. (Un)Fortunately, Buffy shows up and saves Angel at the last minute. She was kinda pissed off so she threw a thingy at Spike which got him in a wheelchair and also set fire to the whole building.

Anyway, Dru is cured but Spike is in the wheelchair (poor thing).
Then, after sharing a very intimate moment with Buffy, Angel lost his soul, joined "The Dark Side" and tried to get Drusilla to himself. I know that he haden't got a soul at that time, but stealing Drusilla from Spike, who was still in the wheelchair, was pretty low. When Spike got some strength back, he asked Buffy for help. Let me explain that: Spike was kinda tired of Angel trying to steal Drusilla from him and Buffy wanted Angel dead so they kinda "teamed up".
Spike was supposed to help Buffy with the Angel-killing part and leave the country with Dru. I'm sure that he didn't expect Dru jumping on him to save Angel. Anyway, Spike won the fight and he and Dru left Sunnydale in his car. After a while, Spike came back to Sunnydale. But he was alone this time. Drusilla had left him and he wanted everyone to pay for it, especially Angel. What Spike didn't know is that Angel's soul was restored. Anyway, Spike thought 'Yeah, why not go to the local magic shop and cast a curse on Angel ?'. When he got there, he found Willow buying stuff for a de-lusting spell. He followed her, and kidnapped both her and Xander. He wanted Willow to cast a love spell on Dru. Anyway, he meets Buffy and Angel and decides to get Dru back by being his old self. That's when he once again lives Sunnydale in his car.