Sarah is our entry point into this crazy world of clones. Born in the UK and raised in the foster care system, Sarah's led a troubled life, and has no clue who her birth parents are—or, you know, that she was designed in a laboratory.
This is her foster brother, Felix:
They met when Sarah was placed in a home overseen by Mrs. S, and they've had each other's backs ever since.
Sarah also has a young daughter named Kira:
Kira's father is unknown at this point, as is much of her early years with Sarah. But we do know that, at one point, Sarah left Kira with Mrs. S in Toronto, feeling she was an unfit mother, and went off to... Well, we don't know. Sarah's reasoning for being in Toronto is to get Kira back.
It should be noted: the fact that Sarah biologically had Kira is an anomaly. It's revealed later in the series that the clones aren't able to have children, so it's not clear how Sarah was able to conceive.
However...plans change when Sarah gets off of a train in Toronto and sees this woman: Beth Childs.
They look alike, don't they? Yeah, Sarah's as freaked out about that as the viewer. Beth takes one look at her, then jumps in front of an oncoming train, effectively killing herself. Sarah is stunned, but doesn't waste any time before stealing her bag. Another thing to remember about Sarah: she grew up on the streets and is used to turning tricks in order to get by.
After doing a bit of investigating into who the woman was, Sarah realizes Beth was rich—Beth's bank account has a lot of money in it. Sarah decides to pose as Beth for a short while to steal her money, then skip town. She also has Felix go to the morgue to identify Beth's dead body as Sarah Manning, so everyone would think someone else died.
It's a fine plan, until Sarah realizes that Beth was a cop, who was actually under investigation for shooting and killing a woman named Maggie Chen. Even worse, Beth's partner notices Sarah's (as Beth) strange behavior, and watches her like a hawk. Also, further complicating things, Sarah begins an affair with Beth's boyfriend, Paul, so he won't suspect anything is wrong...but there's more to him than it seems:
Eventually, Sarah's plan goes kaput, but she's got bigger things to worry about; namely, running into a number of other woman who look just like her. Not to mention, the fact that someone is trying to kill all of them. Oh, and that killer? Looks just like her, too.