I analyze some good ranked sites, they have many backlinks from un-related sites and still they have good ranking and indexing. How much is it important to make the backlinks from only related sites?
It does add quite a bit of importance. That being said though I get backlinks for myself and my clients that may have other content types but are very very useful for ranking on the first page of Google.
Pretty much get back links to your site from other sites that have PR. Note though that your PR will only change when Google updates it. So you may do great link building for a few months but the PR won't show until Google updates it. This can be anywhere between a month to 6 or more months. There is no set schedule.