for the beginning (if you didn't, also others either who might look at this thread) it would be informative to look at this two Eupedia links:
1) eupedia.com/europe/origins haplogroups europe.shtml#mtI
2) eupedia.com/europe/european y-dna haplogroups.shtml
It is difficult, or impossible (even for now, that is, in fact genetics is still in its infancy) to tell who were the Serbs, and who are the Serbs.
Their DNA is not similiar to the DNA of Western and Eastern Slavs.
But they have taken Slavic language. This two things are facts.
There exist few hypotheses.
Serbs today, I think, is a mix of Balkanic (Illyrians, Thracians), Slavic, Anatolian, Sarmatians, Scythians, etc., people, nation, tribes.
Yet, everything of this could be true, and false.
Theories, hypotheses, time will tell, I hope, the truth.
You can read more about the Serbs at this thread:
"Sarmatians, Serbs, Croats and I2a2"
A member "how yes no", posted some very interesting staff.
Take a look, you will enjoy for sure.
And if you have some questions don't hesitate to ask, since discussion(s) and further information can only be helpful.