is there any way to repent for a sin that causes karet? if so, how? and what are the sins that can cause karet?
There are several sins that carry the kares punishment: Violating Shabbos, Yom Kippur, forbidden sexual relations (including niddah), failing to circumcise a child, and so on.
Like all sins, a person can repent for sins that carry the kares punishment, and the main ingredients are confession, true regret, and acceptance never to sin again. However, it is true that the sins are graver than ordinary sins, and therefore the repentance is “harder,” and a person should think about accepting some positive practice upon himself in parallel with his sin, or making a special geder (fence) to ensure he does not return to the sin.