So nofollow links. There're lots of misconceptions about them. The first think you need to understand is that neither nofollow linkes or regular follow links are better or worse than the other. They're for two different things.
As a blogger, correct use of nofollow links is one of the most powerful tools in your box because they allow you to directly affect your site's ranking in a way that few other tools do.
A pop-culture analogy
There's an episode of Community that features a fictional app called MeowMeowBeanz. Google works a bit like that. For those who haven't seen the episode or don't remember it, here's what happens.
Courtesy of Michelle Stein
Each of the characters downloads a smart phone app called MeowMeowBeanz that gives them an average score. They are then able to award the other characters with MeowMeowBeanz which puts their rating up. But, the rating system is exponentially weighted. In other words, the higher your rating the more points you award.
That's basically how Google works; no, really.
Back to reality
Google works in a very similar way. Every time you link to another size, you're endorsing them. If you have a large, popular site, your link will increase the score more than if your site is smaller.
There's a catch though, if you link to a site that is a scam, illegal or of very low quality, Google will penalise you for you endorsement. So it's a double edged sword.
This means you shouldn't use regular links to sites that you don't trust or allow users to generate links to sites (such as in the comments section) that don't use nofollow.
Additionally, you don't want to add regular follow links to content you're being paid to promote. You can't unbiasedly endorse something you're being paid to endorse.
So how to do it
When you're editing your post, switch over to HTML mode. If you've never seen HTML before, don't worry. It can be a bit disconcerting the first time you look at it but it's not that bad. When in HTML mode, you should be able to find the links on your post. They'll look something like this.
If you can't tell, that's a link to Google. Some times they'll be part of a paragraph and a bit hard to find, like so.
<p>Hey! Want a good search engine, why not check out <a href="https://google.com">Google</a>?</p>
Now, what you want to do is modify this link. You need to put the follow code in. It looks like this.
So that your link looks like this.
<a href="https://google.com" rel="nofollow">Google</a>
Once you've done that, when Google crawls your sites, they'll no longer follow the link and therefore they'll no longer pass your page ranking over to the site.
So when to use it?
When you're being paid to link
In order to protect you reputation, Google encourage you to add nofollow links to link you're being paid to put up. You certainly shouldn't charge for links.
When you don't trust the content on the other side
If you have a comments section on your blog, you'll know that people tend to comment with links to their own blog or worse a spammy/scam site. Your site should automatically nofollow these links. You can right-click and click and "Inspect element" to check.
When the link goes somewhere google doesn't understand
Some pages on your site Google can't possibly comprehend. For example, the log in and register pages if you have them. You should nofollow these links because Google is not able to register or log in to your site.