If you are wanting to target a niche via blogs this is how to do it.
1. Do keyword research.
What you are looking for are keywords which get traffic, which do not have very high competition for ranking, which are relevant to your niche.
2. Register your domain.
Register a domain that has a 100% match to your keyword. .COM is best but .NET and .ORG are ok. For example if your keyword was "MILFS LOVE COCK" then you want MILFSLOVECOCK.COM.
3. Install Wordpress.
Install WordPress onto the domain and immediately write 200-300 words relating to the keyword. So using the example above, you will want to write a nice post about MILFS who LOVE COCK. Doing this allows any search engine crawling your site to see what your site is about.
4. Sponsors
Join the sponsors that best fit your niche. Make sure they are sponsors which are known to pay and have good support. I recommend sponsors like
Pimproll which have a great range of sites in different niches.
5. Write a minimum 5 blog posts per week for four weeks then write a minimum 3 blog posts a week from then on. Do keyword research beforehand and target those keywords using them in titles and blog posts. Use nofollow links on your affiliate links, keep those blog posts at 150 - 300 words minimum. Include at least one image per blog post. When you get more confident use plugins like NextGen gallery to add galleries to your posts.
6. Repeat this cycle often. Remember something which you do which earns $100 a month will earn you $1000 a month if you do it 10 times. Stick to basics. Don't spend money on traffic, spend time on writing.
For keyword research there is only one tool I recommend and that is
Market Samurai. If you can master this tool then everything will be much easier.