For getting the latitudes and longitudes of any major town in the world one can either refer to some atlas or the same are also available on the websites given below

http://www.heavens-above.com/selecttown.asp

http://www.astro.com/cgi/aq.cgi?lang=e

To convert minutes into decimal, kindly see the following example:

60 minutes correspond to 100 decimal. So, if a particular town has
Co-ordinates - 36o 28" N and 80o 30" E
then in decimal it would convert to
60->100
28-> ?
28x100 / 60 = 46.66
36. 47.

60->100
30-> ?
30x100 / 60 = 50
80. 50