You'll get there.
If you want to recreate what I did you need to turn off the environment in the render settings, leave the dome on but zero out the environment itself:
View attachment 144920
That will give you a solid black background so you don't need to add more props to handle what shows in the mirror. You'll have to add some more lighting depending on what results you want.
For the floor, I dropped the base color down to black so you only see the wood grain in the gloss coating. That way to floor fades into the black background and it's essentially invisible in the reflection due to the light placement.
With the long T-Shirt you're using you can get a cool effect by increasing the translucency of it and messing with the shirt's SSS settings. With the character between the camera and the bright light around the mirror you'll see her silhouette through the shirt in the foreground.
And, for portrait style renders try to put the camera where you want the eye height of the viewer to be (normally close to the eye height of the subject of the picture) and then angle it until you have it framed the way you want. That will make it look more natural rather than having it as high as you did.
Hope that helps.