First of all, we find Cain's genealogy laid out in Genesis 4:16-24.  Verse 16 is very interesting, especially the first few words: "And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden."  It seems that when Cain chose to murder his younger brother, he basically separated himself from God and his family.  As the firstborn, Cain forfeited his inheritance to Adam's birthright (for another example of this, see Genesis 25:27-34) and this explains why Cain is not listed in the genealogy of Genesis chapter 5.  Notice that this chapter only lists one specific son (the firstborn, with the exception of Cain) of each descendant of Adam.  For example, look at Verses 6-7:  "And Seth lived an hundred and five years, and begat Enos: And Seth lived after he begat Enos eight hundred and seven years, and begat sons and daughters."  Here we see that the scriptures highlight his eldest son Enos, but the rest of his siblings are not mentioned by name.  Keep in mind that these people lived in a very patriarchal society, meaning that the oldest son was to take over his father's role as the leader of the family and the whole entire tribe.  Cain missed out on this opportunity and that's why he's not included in Genesis chapter 5.

As far as the New Testament is concerned, the only genealogies listed here are those of Jesus Christ Himself.  The Gospels writers traced a direct line from Christ all the way back to Eden.  Cain should not be a part of this lineage because instead, Jesus descended from Seth (Luke 3:23-38).  Again, not every family member is given, only those who are direct relatives; otherwise, this would be quite an enormous list!

Always remember that the New Testament advises us that we shouldn't get too wrapped up in genealogies, but that we focus more on the spiritual priorities!
(See Titus 3:9 and 1 Timothy 1:4)