What if the Watchers never tracked Buffy down for one reason or another? And what if Spike somehow found her? 

Buffy has no idea she's the Slayer and continues on with her life as normal. She notices that she's better at cheerleading and that she's stronger than she used to be, but she doesn't really pay much attention to these facts. 

Then one night her life changes in the form of a bleach blonde, punk rocker approaching her after cheerleading. He keeps calling her 'Slayer', refers to himself as the 'Slayer of Slayers', and talks about vampires as if they exist. Buffy is creeped out, obviously, which turns into major freakage when the guy's face shifts, revealing that vampires really do exist. 

Spike is in town because Drusilla dumped him, and he figured that killing a Slayer would win her back. But he knows that killing a Slayer who doesn't even know she's a Slayer isn't fair play; a Slayer deserves to go out in a blaze of glory. This Slayer would barely even be worth it; no good fight at all. 

So at first decides to back off and wait for the Watchers to seek her out, only they don't, and when fledglings keep on attacking Buffy - who has gotten over her initial fear of him and keeps following him around like a lost lamb - Spike reluctantly takes it upon himself to give her some form of training with the goal of fighting her when she's ready. 

Does the thing with Lothos still happen? Does Buffy still burn down the school gym? With back up from Spike, does she tell Joyce about being the Slayer? And what happens when they re-locate to the Hellmouth? 

What happens after this is up to the author, only the relationship between Buffy and Spike has to be more than "Buffy's first love" (as in she has to fully understand what she's getting herself in for, something that didn't happen with Angel). 

Speaking of which, Angel isn't called up by Whistler, and continues to feed on rats in New York. Whether or not he shows up at all in this story is up to the author.