Super affiliate here. While Everett and Thomas are right to point you in the direction of the FTC texts, keep in mind that the FTC guidelines are just that: guidelines. They're not law, not widely enforced, and Google largely ignores affiliate links so long as they're nofollowed. They don't seem to be quite up to the level of the sponsored links and content that bloggers must disclose to avoid a Google penalty (but again, that's Google's enforcement of their own guidelines, not law.)
Some merchants with affiliate programs will insist on clear and conspicuous disclosure if you want to remain in their program, but really their panic over the guidelines died down pretty fast after they were released in 2012. The one major exception that comes to mind are credit cards providers, which are exceptionally picky about compliance on this issue since they're subject to tighter regulation on the banking side of things.
All that is not to say that disclosure isn't the right thing to do - only that it's not likely to land you in hot water with anyone anytime soon.

