The black region seen is due to tooth decay
Next Steps
You will need to visit a dentist in order to get your tooth filled with an aesthetic filling material preferably composite
Health Tips
Also a word of advice, after the filling, floss regularly in the area to prevent further decay.
You can also consider getting orthodontic or braces treatment done in order to correct the alignment of your teeth as malaligned teeth can lead to increased food deposition and subsequently dental caries