I've decided to make this thread so that people who know the answer to where you find the benefits and rewards can tell the ones who seem unable to find them (I.E. Me :P)

Here you can find a nice guide to the factions (it is free):

http://www.dmsguild.com/product/199498/A-Guide-to-the-Factions-of-Faerun?filters=45470_0_0_0_0_0_0_0

Partecipating actively for a faction can basically provide more roleplaying tips, such as:

safe places; general or specific information; magic item (including spells, potions...); aid for quest

The help from a faction doesn not come free (they may want something in return) and the whole thing is always at the DM's will.

I really like this post and this post on this subject. It's not exhaustive, but provides a really strong basis for developing your own ideas.