I just did the Discover Goku quest ore something.
So I did everything you see here:
JoaGLmyPSnY
But after that I the old guy came out his house again and I talked t him...
And now I can hit him but can't do any damage?
What Am I supposed to do?!

your suppose to charge RP. think its Z its Z on korean game not sure about this.

thats exactly right, you can actually leave right after you get the skill.

What after this tho wer do i go

You leave after that.

I charged and then he's still attackable , lol?
That's a bit confusing...
But anyway I left like you said so it's done now.
Tnx for the help everyone.

You have to use the point you get from charging with one of your attacks on him.

I used all attacks I got xD
But anyway i left now and I don't know how to go back so it doesn't matter.
/close topic.

Once you hit Grandpa Gohan successfully with a powered up attack (Holding "Z") you should get a "check point" arrow. This is the end of the sequence.