I can't help you with the beginning of your story. That has to come from within you, not me. The reason for this is that many things depend on where you start. However I do have a few suggestions. if Laura is unaware that Dennis and Rita are not her real parents she would have the same last name as them. So her name is not Laura Jones. She has their last name.
She might not be named Laura if that is the name she was given by her real parents. She might be named Susy Smith assuming Smith is Dennis'' last name. But she will not be living under her birth name. She will not have a necklace that she had in her old life. She doesn't have the clothes she was last seen in. In brief, she will have nothing physical to hint at her old life.
I have vivid memories of events that happened when I was four. I couldn't write a book about it, but I remember many things.
She is watching a show about missing children? Hm. If you research this you will find that stranger abductions a more rare than you think. I'm not saying it doesn't happen. But it is not as common as we are led to believe. However abductions by the parent who loses a custody battle are not unusual.
Perhaps the show is about parents and children who go missing, or just about people who go missing in unusual circumstances. few years ago I saw a show that covered this teen who went missing. They found his car hundreds of miles from his home. There were two books in the car. Into the Wild and The Catcher in the Rye. You might have to be a male of my age (sixtyish) to understand how scary that is. Finding those two books, I mean.
Her real parents don't have to have been saints, nor do Dennis and Rita have to be creeps. If somebody stole Laura in order to sell her to Dennis and Rita it is more likely that Dennis and Rita will be very loving. Another thing about D & R is that when they had friends over somebody wold comment on the lock on the living room door and ask about it, and why would they lock that door in the first place? To keep Laura from watching TV? She will see TV at her friends house.I s Laura haunted by memories of somebody in her past who was especially kind to her? Was there some little ritual that they shared? Say Laura likes apples and there was a kind man (father or grandfather) who would try to peel apples in one long strand.
Maybe Dennis or Rita is her loving birth parent. They lost the custody battle. The parent who won custody battle is a danger to Laura (so far as the other parent is concerned). Or, the parent who won custody decided they needed to flee with Laura because the other parent was a danger to Laura.
So why does Laura Jones think she might be Susy Smith?
Let me give you an example from my life. I have two scars that are not as visible now than they were twenty years ago. One is on my chin. I was running and fell and cut my chin open. It was quite a pain to shave when I was younger. And I have a long scar on the inside of my upper right arm. Very noticeable.
On the TV show the presenter shows and describes visible scars that might help in identifying the missing girl. Laura thinks this is odd. She has those same scars. She grows curious. How could two people share the same scars?
Just for fun she looks into her own background. And things look odd. So she digs further. The internet is a great tool for finding things out. She searches for the missing girl. There is a website. What she finds makes her more curious.
What does she find? That's up to you.
I'm not pointing these things out to be mean. But the story as you have presented it is not thought out well enough. I think you can do better.
I also need to point, that you, have a tendency, to use, commas, when you don't need to do so. It's better to use too few than to use too many. I have the same tendency myself.
At some point somebody is going to point out that this idea is not all that original. Don't worry about that.. Nobody has had an original idea for the last three thousand years. Whatever you come up with some ancient Greek already used it.
But your presentation of various ideas can be original.
Good luck on your story. It makes me happy that there are people - especially younger people - who find it important to tell us stories. The story-teller is always worth receiving our gratitude.