The key to developing profitable websites or domains is good content. The first thing you'll want to do is make sure your domain name is about something relevant or interesting. There is no point in developing a domain name that is of little or not interest to anyone.
Focus on your domain names with strong keywords and good Google search popularity. Easiest way to check the popularity of your search terms is by simply typing them in quotes into Google. See how many relevant pages are returned - if under 100,000 then your topic is not very popular - over a million and you may have a domain with some good potential.
When developing your site make sure to provide visitors with useful information. Rather than simply being a portal - make your site a destination.
Don't use low-quality templates, if you can - develop the site yourself. Tools like Wordpress can help reduce your development time if you know what you're doing.
Add monetization that makes sense for your site. Google Adsense is a great place to start. Put yourself in your visitors shoes - what would you be interested in?
Don't think of just throwing together a site - instead put some time and energy into and you can develop a nice growing investment.
Hope this helps! PM me if you would like some examples of sites I've built.
Best,
Morgan