Almost every business on the internet has some kind of referral reward program, not just casinos and poker sites.
If you want the LONG story, see
The easiest way to start becoming an affiliate is to pick a few sites you know and trust and then open their web pages.
At the top or bottom of the home page should be a link to the 'affiliate' pages.
Generally you simply open an affiliate account using your current login details for that site, answer a few questions about what you will be doing and where, plus how you wish to be paid, and then they create a unique affiliate url for you (eg casinomeister.com/affid=Roanan).
These links will also be affixed inside the affiliate banners they make available for you. These are usually in an html format and all you do is copy and paste your unique codes and place them where you desire on YOUR web page.
Alternatively, there are
such as Commission Junction that is a central hub for affiliates to explode their banner farm, giving you a central resource for all types of online businesses looking for affiliate, not just casinos, as well as a central source for commission payments.
What you need to do is figure out if you want this to just be a sideline source of income, or do you want to do this for a living.
If it is the former, then this should start you on the road to a nice small, steady income.
If the latter is what you wish, and you have the time to devote to this, then I suggest you send the link for this thread to Bryan or some of the many affiliate managers that are active here, and they could probably help you get started on something bigger.
Addendum: For all of you out there who have any kind of page at all online, whether it be a blog or their own website devoted to something else entirely, I strongly
encourage you to do the something along these lines..
I have always had an UltimateBet, PokerSpace or BankrollMob banner or link on my pages.
Even my Facebook profile has one
The extra $50 that lands in my MoneyBookers (even they have an affiliate program) account every so often is always a nice bonus for basically doing nothing, and it only takes a moment to set up.