It kind of depends if you are planning a domestic or international adoption. Most countries outside the US have minimum age requirements of 25–30, just depending on the country. Some countries will not allow a single male to adopt.
The US infant adoptions – while legally you can adopt at 18/21, most agencies are pretty strict with their requirements too. Mostly I see 25 being the minimum age, and they prefer a few years of financial stablility to consider you as a prospective parent. Doesn’t mean you have to be rich, just means you have to have a stable consistent income.
If you are considering adopting a child from the foster care system, I think a lot of states have an age of 21. But again, they’ll want to see the financial stability, emotional stability etc.
As for resources, it would help if I knew what type of adoption you are thinking of doing. There is a ton of info out there specific to domestic, international, fostercare etc., so helps to narrow it down.
http://www.adopting.org/adoptions/learn-about-adoption-who-can-adopt.html