Hello. Firstly, you are only 20 year old and wisdom tooth extraction is not generally advised until there is patient discomfort that can not be treated otherwise.
Secondly, since you have got it extracted, it is very important to follow the oral hygiene instructions given to you by your doctor as this helps in better and quick healing of the wound.
Thirdly, the white thing you are mentioning can be food slough over the wound, but only if it is not excruciatingly paining you.
Next Steps
Any symptoms, then it is good for you to consult a specialist doctor i.e., Oral and Maxillofacial Surgeon.
Health Tips
Do brushing softly and warm saline water rinses every 2-3 hours.