I am using a tool that says the Nairaland links are all do-follow. I want to clear one more thing, it does not mean that it passes all the link juice to any other website and losing rank on Google.
Nairaland is a highly authoritative site. Google wants links, Nairaland passes more links to other sites, which means it is more relevant to those contents. The possibility will be higher for Nairaland to rank for a variety of keywords. If you check, you must see this site has a low spam score. That means we, the users of this site are not spamming.

Please learn more before you start, personal effort will be more effective than suggestion.
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How can Joomla I learn from here?

I have written about SEO on my website and about simple tips and tricks about many things. See if you want-
Free Online Tutorials are available on my website. I have adsense on my site. I can't earn from adsense. Please give me some suggestion regarding this.

You say it gives 7$ for 1000 impressions. What is the condition for a website to use their services
How do they make profit?

Yes, it is not so tough to do that. Primary ideas about HTML is enough to do that. I did it myself for my site.