How can I give relation once content type to other. Eg If I have a product and the product sub items. I need to make a product content and a product_sub_item content type. Also need to select the product while creating the sub item. How to achieve this relation
Check out the module "References". It allows you to create a field where you can reference on other content types and nodes.
In addition, you might also want to take a look at the module Relation which looks to have a brighter future in terms of active maintenance and integration with other Drupal contribs. Another module that will come handy is Rules.
For both modules there are good introductory screencasts available by Johan Falk on http://dev.nodeone.se/sv
I discuss some of the different options available here: http://drupal.org/node/1293792#comment-5275780
This discussion is particularly relevant if you care about back-references. For your use case, perhaps references & cnr would give the best user experience.