I have a fore skin on my penis. Whether it is good or ashamed

It is neither; some people prefer either one, because of their religion or not.

So what is thd advantage

The advantage is good hygeine; if not cleaned well under the foreskin,material will accumulate which causes infectiion which makes pulling the foreskin back very painful. Read more here

= Easier hygiene. Circumcision makes it simpler to wash the penis. Washing beneath the foreskin of an uncircumcised penis is generally easy, however.
= Decreased risk of urinary tract infections. The overall risk of urinary tract infections in males is low, but these infections are more common in uncircumcised males. Severe infections early in life can lead to kidney problems later on.
= Decreased risk of sexually transmitted infections. Circumcised men might have a lower risk of certain sexually transmitted infections, including HIV. Still, safe sexual practices remain essential.
= Prevention of penile problems. Occasionally, the foreskin on an uncircumcised penis can be difficult or impossible to retract (phimosis). This can lead to inflammation of the foreskin or head of the penis.
= Decreased risk of penile cancer. Although cancer of the penis is rare, it's less common in circumcised men. In addition, cervical cancer is less common in the female sexual partners of circumcised men.