In the ActonTaken tab put this kind of script when the quest is taken
void main()
{
string sTag = GetTag(OBJECT_SELF);
SetLocalInt(GetPCSpeaker(), sTag, 1);
}
In the StartingConditional of the next node of your conversation put this one:
int StartingConditional()
{
string sTag = GetTag(OBJECT_SELF);
if (GetLocalInt(GetPCSpeaker(), sTag) == 1)
return TRUE;
return FALSE;
}
Note that the node with the conditional should be set before the node where the quest is taken in the conversation. Also note that these scripts are generic and can be reused with another npc (as long as their tags are different!)
Good luck!