The first key to the success of a blog or mini site is that it has to target a group of people (a niche) who are hungry for information of a specific nature. In other words, I'm talking about the selection of your niche.
The group of people that your blog or mini site targets has to be a group of people who have problems. Problems in the sense that they are looking for a solution to something, looking to improve something. Furthermore, in a great niche, that SOMETHING, is something that causes pain. Financial pain, physical pain, emotional pain, some sort of pain, for the simple reason that people who have pain are the people who will want to buy solutions to that pain, and hopefully from your website.
If that sounds a bit sick, trying to exploit people with pain, it's not. It's called marketing. You've had marketers doing it to you your whole life and you probably never even knew about it, why can't you do it too? And what's more, you're actually going to provide these people with a real solution, so you're being helpful!
Anyhow moving on.
So you're looking for a group of people with a problem, but that's not all.
Your next concern has to be whether there are enough of these people searching for solutions to their problems on the internet. How many is enough you ask? It's a good question. With certain kinds of sites, all you need is 10 visitors to your site a day and you can turn a nice monthly profit. Other niches and products that you promote you might need thousands per day. It all depends on what you sell, and who to, AND how much commission you make when you sell to them.
The things you should do are look at the search volume, as indicated by a tool like Wordtracker. Here you're simply looking at how many searches per day a particular keyphrase gets in the search engines. Then after that, you want to look at whether there are other sites or blogs that are competing in the same niche.
Contrary to what alot of people think, NO competition isn't necessarily a good thing. NO competition could and often does mean that it's not a profitable niche. What you actually want is SOME competition, but CRAPPY competition that you can DESTROY by making a better site, providing better content and in a better way.
The final piece of the puzzle of finding a good niche, is whether the niche has a quality product for you to promote. You need something to sell to these people, that is high quality, has great marketing material (sales letter etc, so it will convert your traffic well) and pays a decent commission.
This is of course assuming you're looking at the affiliate marketing model of making money with your blog - which I suggest if you're looking to retire from your job and earn a full time passive income, you SHOULD be. There are other ways to make money where you might think about some different factors in your niche selection.
So if you can combine all of the above factors when selecting your niche, you'll be giving yourself and your new blog the best possible chance of success. I hope you've found this valuable.