Google spider is very intelligent; it has the power to choose quality websites and show them in a search result.
I will not explain in details what a backlink is, but I will share with my readers what is important for search engine optimisation.
The first thing that you have to do is to contact webmasters that have a website where you can find the same content with same keywords of your website; if they have an high pagerank is better for you, but if they have a good pagerank with different content from your website, do you think that google will not know this? Your high pagerank backlink will be not important for your ranking.
An other important thing is the anchor text of the backlink; be sure that the text and the tag alt or title contain the keyword for your search engine optimisation.
Forums and blogs are good way to build backlinks, but unfortunatly you will have the tag nofollow with your link; it will not increase your ranking for search engine, but you will have target visitors if what you wrote sounds interesting for them.
Please continue this post with your knowledge; thanks!