There's a ton of affiliate networks and programs so you need to know what you want to do first.
If you're new to affiliate marketing, you need to decide if you want to sell stuff or get people to fill out a form. To sell stuff, you can start with ClickBank cause it's a joke to get accepted with no experience. Or if you want to sell physical products then go for Commission Junction or LinkShare, but the good advertisers won't approve you unless you have something up and running and making money.
For getting people to fill out forms like email submits, search for "CPA affiliate networks".