Well first things first, if you are Here at the forum you probably know what amateur radio is.
But if not you can check out this link,
http://en.wikipedia.org/wiki/Amateur_radio this will give you a basic understanding of amateur radio. Next you can do many things to expand your knowledge including acquiring text materials on this subject.
This would include things such as QST magazines or brochures, there are also many books on this subject. One of the newest that comes to mind is "amateur radio for dummies", this book brakes down the hobby into plain English and basic comprehension.
Or you could simply pick up an ARRL Technician class test manual, which will include theory, rules/regulations/guidelines, and practice exam questions. This will prepare you to obtain your "Tech" license.
There is also another avenue you can take, which is to take a class for your exam, some are free and some are not. Check out this link
http://operationteam.org/about.html this is an example of a group that gives classes. If you are short on money there are study materials that can be found for free on the internet, for example this link
http://www.ncvec.org/downloads this site has the complete question pools for all three Amateur radio exams. Now this does not include the theory however if you wish to obtain the theory it can be found on the net as well. Once you you have obtained your study materials you can take practice exams at this link
www.qrz.com There are three current classes of licenses in the USA and they are as follows.
Technician Class- basically an introductory license. This license is obtained by taking a 35 question exam.
General Class- is the next step up and gives you more privileges and teaches you many more things, as it prepares you to be a better operator. 35 question exam
Extra Class- Is the highest license class in amateur radio and gives you full amateur privileges. 50 question exam
Now once you reach the stage of general you can apply to be a VE (volunteer examiner) however this limits you to giving tech tests. In order to give general and extra tests you must be an amateur extra with a VE license
Well that covers the basics I hope this was helpful and encouraging to all who read it.
For those of you planing on becoming an amateur radio operator or upgrading your current class good luck.