If you have purified DNA then you just need to make an appropriate dilution in buffer and read the wavelength at 260nm to quantify the total nucleic acid. Note that you can not determine only DNA, this will be DNA and any RNA that might also be present. Be sure you use a buffer only blank to "zero" the machine and especially be sure you use a quartz cuvette that is appropriate for the UV spectrum. Cheaper glass cuvettes do not permit UV wavelengths to pass through adequately and you need to use the more expensive quartz cuvettes.

You don't really need any special software. A solution of double stranded DNA of at 50ug/ml gives an absorption of 1.0 at 260nm, so take your reading and calculate accordingly to determine your stock concentration. You can get full instructions here.

http://cshprotocols.cshlp.org/content/2007/11/pdb.ip47.full

Some people do the ration of 260nm/280nm to determine the purity of their DNA stock. Protein absorbed better at 280. A ration of 1.8 generally connotes pure nucleic acid, below 1.8 means you have significant protein contamination.

Find out the absorbance of Extracted DNA at 260 nm. It is assumed that Absorbance of 1.0 at 260 nm = 50 nano gram DNA/ml. So if absorbance is 0.5, the solution contains 25 nanogram DNA/ml.