I'm not a pro but I will try to do my best to help you. I understood you just took that picture for testing purposes and that you simply want to know how to improve this particular photo using Lightroom. People already indicated that the exposure is not the best here.
My workflow for this would be:
- set the whitebalance (try to use the door in the background) - noise will look less disturbing in my opinion
- set the sharpening amount to 0 (will do sharpening after noise removal)
- There is color noise in the image. Zoom to 100% and move the color noise slider until it disappears. (dark areas have more color noise)
- Set luminance noise reduction to at least 80 or even 90. Yes you will loose detail. You can't avoid that at ISO12800.
- set sharpening amount to maximum to begin (will decrease that value later)
- This scene has no fine details, so you can use a much higher masking value in the sharpening tab. Press and hold the ALT key while moving the 'masking' slider up. You will see white lines. The correct masking value will be the one when you can only see the outline of the edges of the door and the wall.
- 'detail' will try to bring out detail in that shot. There is not much detail but straight lines. High detail values increase noise. I'd set it to 1 here. You could also press and hold the ALT key and look at the picture at 100%, too much detail will emphasize the noise.
- My feeling tells me a radius of 1.2 is okay here (again, try to press and hold the ALT key)
- Set the sharpening to about 90 or any value which you think is enough. Pre-sharpening can only be seen properly at 100%.
- Export the image. LR will add output sharpening to the picture. This step is important.
The workflow above will look weird for photos of people. They will look like if they have a skin made of wax. It might work for this image here tough.
Question is what you are planning to do with the pictures. Print them? Publishing on the web? Pixel peep for noise? It also depends on your viewing distance / size of the final picture. The settings would be different for each.
There are also very good 3rd party noise removal tools like Nik Dfine 2, available for download for a 30 day testing period.
Sorry for my clumsy english - reading is not an issue but writing is hard :-)