How would I start a scene where I show the character past can anybody help me??
You could do a transition into it and have it in a different color, like black and white or sepia. I also usually have a narrator box come up that says like x years ago or whatever. Really however you want to do it.
I prefer using a white transition and a filter (black and white or sepia) or if you are not using filters, establish on a reader message like (readerMessage This is a flashback!) or
This is a flashback!