20 minutes after the wedding ended...
Lyra and Bon Bon finally arrived home after giving Cranky Doodle and Matilda a congratulations on their wedding.
"Whew, that was a fun wedding, wasn't it, Lyra?" asked Bon Bon, closing the front door behind her.
"Yeah, I know, right?" answered Lyra, who was sitting on the couch in the living room, munching on some hay fries she had decided to take home from the wedding banquet earlier. "Um, Bon Bon?"
"Yes, Lyra?"
Lyra then asked Bon Bon a question that has been bugging her for quite a while after she found out that she was a secret agent, "Yeah, about the whole secret agent thing, what do you think will happen if you had accidentally blown your cover and your agency found out?"
Bon Bon knew this question would come up so as she gave her best friend a sad look, she said, "Well, I would probably get relocated someplace far away, but that probably won't happen as long as I keep this a secret." She then gave Lyra a stern look. "You promise to keep my double life a secret?"
"Of course! I would never do that to my bestest best friend in the whole world! Even if you are a secret agent, that will never change my opinion on you. So, cheer up, okay?" Lyra answered with a huge smile plastered on her face.
Bon Bon's expression changed a bit as she breathed a sigh of relief, "Okay, I trust you, Lyra. Now if you excuse me, I need to go take a shower. It's getting kind of late."
It was then that the mint unicorn noticed that the sky outside is really dark. "Oh yeah, I need to shower too, but you can go first if you want to, Bon Bon."
"Aww, thanks, Lyra! You're the best!" exclaimed the earth pony, walking up to Lyra and hugging her before making her way upstairs to the bathroom to get herself cleaned up.
Unbeknownst to any of them, an unknown pony was peeping outside from the kitchen window since the living room and the kitchen are joined together. The pony then gave a huge grin on her face as she slides away from the property and headed off to make the preparations needed to finally get hold on Lyra as her new girlfriend. It may take a while but it will all worth it in the end.
"Just you wait, Lyra. Soon, we will be together forever after you leave your so-called best friend behind and replace her with me."
It's an interesting start, though it feels a bit rushed. As this is the basis of your fic, the first introduction which either draws in or pushes any potential readers away, you may want to flesh out this chapter a bit more. It is a short chapter, so it should be a simple affair to add a bit more detail here and there.
For example. Here's a part of this chapter:
It works well in pushing the conversation from one topic to another, but it feels a bit out of place. You could add to it by mentioning the difficulies Lyra has with the discovery of Bon Bon's secret life.
Example:
Of course this is my take on things, and this is only a suggestion. You are the master of your work, and should not feel pressured into changing anything you don't feel comfortable with. Also, seeing how this is your first story, and that you have only begun with it fairly recently, I understand how easy it is to overlook things like this. The first things I wrote were on a level of crap that the only times you would even notice it, is when you look down to see what it is you stepped into.