Can anyone tell me how to get back links from the high PR sites?

Hello..
start link building there but these should be relevant,then it will be beneficial for you..

Yes, nowadays relevant link building effect only !

yes it really matters alot after so many panda and penguin updates it is mandotory to build links which is relevant to your site  category.

All Forums has a genuine community for discussions and exploring many ideas so getting backlinks from forum will not only bring new visitors to your site/blog but also gives a strong backlink for good SEO.