It's not trash, but it's not easy to earn through it. You need lot's of traffic on your website/blog, so that more people click on ads. But in general there are better ways of earning online. Try affiliate programs(Amazon i.e.), like when you read some book and you like it(or not), you can put out it's amazon link on your site, and if someone who buys it came from your site, you earn $.
One's who earn most from such programs create search engine optimized(SEO) websites so that they rank high in search engines, and get targeted customers(people who search something exact and get that at their site).
There are also A LOT of scams in internet marketing so watch out.. don't pay for anything that promises you $$$.