It is not necessary to register on such forums and insert your links in the first comment. You can be banned and won’t get any sense from that link as it will be deleted. If you post a link on forums make sure that this link is dofollow. What is really great is to create your own forum and take an active participation into forum discussions.
What concerns blogs, read only topical and interesting articles for you where you can really leave comments with your link that will help people to get additional information, cast back author’s opinion or just give some advice about something.
Leave comments on blogs with good authority and traffic as these kinds of blogs will give significant results. Try to choose dofollow blogs, but you can vary them with nofollow blogs, of course, if a blog site is really worthy. You should notice that most of comments are moderated that’s why you should write more extended and good comments, but not just “Thank for your article” or “It is a really informative article” as you can just like or tweet and the author will know you like his article.
Don’t spam and post comments on non topical and low quality websites in order to get more backlinks on your website. Make sure if you have a lot of spam links you can get Google penalties.