2. Find some people present at that time to write some certificates, the main content is that Yang caused your father's injury;
3. It is best if you can copy some written materials asked by the police.
The above evidence has a direct causal relationship with your father's injury and Yang's behavior, and has caused serious consequences (minor injuries).
It is best to ask the other party for compensation when filing an incidental civil case, such as medical expenses, lost time and nutrition expenses.
If the injury is minor, the public security organ may not accept it, but it shall inform the victim that he can file a criminal private prosecution with the court. However, your condition is a facial injury, which may be more than a minor injury.
Cases of minor injuries are handled only after notification, and courts generally tend to mediate. If the defendant pays better compensation and gets the understanding of the injured party, the sentence will be lighter, and he can be sentenced to probation or acquitted. If the defendant refuses to pay compensation, the court will give corresponding judgment according to the circumstances of the case. The former case is probably a public prosecution case, and his situation is different from yours.