Hi Hisham,

It depends on what you want to do. if for example you want to analyse both DNA and RNA from the same cell culture or tissue you can use this kit from Qiagen (AllPrep DNA/RNA extraction), it allows to get DNA and DNA from the same sample. otherwise you can use phenol-chloroform extraction as suggested by John or other dedicated kits; personally I like the first option, even a bit laborious, because you get DNA of greater quality. For RNA you can use either dedicated kits (from Qiagen or Invitogen for instance) or use Trizol (from ThermoFisher) or RNAble (Eurobio); the last ones are recommended because you get RNA of good quality.

Hope it is helpful