CJ is one of the most complete affiliate network, you have to join the program of each separate company, and then you will get a lot of promoting tools from links, pop ups, baners etc..
Click bank is a marketplace for individuals who sells products, such as videos, tutorials , food, anything.
This a good tech. network.
This is the most famous, you join and you can search from the affilaite system for products, you can then get your hoplink or banner.
One of the most complete adult affilaite network. Selling adult product might be the easiest way to start off.
There are a lot more of course, just type your niche and affiliate network on google and you will get more, those above are the most complete and famous.