As far as making money on a site, you never know if a site will be a winner or not. You are going to have winners and losers. Trial and error will become a part of your day.
Build a lot of good content and promote your site. It helps for your site to have been up for a while.
You will need an attractive site with
SEO, a lot of good content and a niche site that has little competition.
Don't build a site that you are going to have a million sites (literally) to compete with.
Come up with a unique site on a different topic.
For instance, I saw a site the other day that was about puppets. I thought that was really unique and that it wouldn't have a lot of competition.
You will have to "Think Outside the Box" to come up with a good niche site.