If this question has been answered before, please, point me there and delete this post.
But my question might be a bit deeper than its title. Some time ago on STM I came across reference to this article - http://gulyani.com/complete-list-of-...rks-companies/ - obviously that's a big list and number one issue is mostly trust.
How do you decide who to choose for your next test or campaign? What makes you trust the new traffic source or a network with new offers? Is it payouts, deposit sizes, CPM, CPC etc. rates? What features do you look for before clicking that signup button as advertiser or a publisher?
Traffic source - Depends on niche. Find the big ones in your niche, start with a few particularly reputable ones (should be easy with a bit of research). Figure out their inventory, see if other affs are running on it and if you think what you want to promote fits with their traffic. Test. Repeat.
Affiliate network - If you don't know who to trust, a little research on STM give you all the info on good networks you need. If you know who is trustworthy but just don't know who to pick, sign up for a bunch and see which affiliate managers you end up getting along with best. You'll often work closely with AMs so if you can find networks with good AMs you really like this will make things easier.
It also often comes down to exclusives. Some networks have offers the others don't, or much better payouts, so in these cases there isn't much choice.
Dusklife nailed this. There is so much information on STM covering the best/trusted CPA networks and of course when starting out in a specific vertical try learning the ropes in one of the favourites to get your feet wet.
Social - Facebook/POF
Mobile - Decisive/Leadbolt/Airpush
Adult - Trafficjunky/Exoclick
etc let us know what you're looking for in an affiliate network offers wise, and what vertical you would like then we can help direct you