Well do you want the short answer or to take a 3 month course in genetics.
If you just mean how does DNA replicate itself, it splits apart into two chains of DNA, this is fairly easy to do as there are weak bonds between the base pairs.
The base pairs
Adanine
Gaunine
Thymine
Cytosine
Click to Enlarge.^
May have seen a code like this before to express say a gene
AGCTCTCCCTAAGGG... etcBecause Adanine only pairs with Thymine and Gaunine with Cytosine. The free base pairs in the Nucleus will pair with these to form two strands and two copies, one of which then breaks off.
AGCCT
¦¦ ¦¦¦
TCGGA
I haven't gone into the enzymes and how they achieve this or any great detail, to be frank it's an complicated process, but simply speaking this is how DNA is replicated. As for how it's made, or how a cell splits into two complete cells: mitosis, or what it's made from, a Google search will help there.
http://en.wikipedia.org/wiki/Mitosis"
http://en.wikipedia.org/wiki/DNA" Once these chains have been replicated from the DNA nucleus, single strands of DNA can then be used to encode proteins in various areas, I could explain how they do this by using tRNA and mRNA (transposeRNA and messenger RNA) but this is another long winded process involving 3 base pair codons for each protein.
http://web.jjay.cuny.edu/~acarpi/NSC/12-dna.htm
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Codons.html
Here's a list of all the codons and there respective polypeptides(the building blocks of proteins)
You may want to be a bit more specific, this is a large area and a complicated one.