How to learn e4 e5 as black? I'm about 1800 FIDE and I wanted to incorporate e4 e5 to my repertoire as black, however, as soon as i started studying it, it became clear to me that its one of the most complex openings to prepare.

So can you guide me to the right approach in learning this opening? What are the best resources? Where can i learn the ideas instead of just memorizing lines? Do you have an tips regarding this opening? And who are the best practitioners or e4 e5? 

much appreciated!
There are a lot of good resources on 1. e4 e5 lines.  I would recommend you combine some study of an up-to-date theoretical work (Bologan's two books, Black Weapons, and Bologan's Ruy Lopez are very good, and all you would need for a long time) with going through some games.  For example, if you want to get a good feel for where your pieces belong and typical maneuvering in closed Ruy Lopez positions, going through Karpov's games as Black in these lines can teach you a lot.

Ultimately, you are going to have to memorize some sharp lines.  Many of the gambits played in the Romantic era in chess come after 1.e4 e5, and it is simply not possible to play these positions correctly without learning some theory.  The good news is that Black usually has a number of sound options against White's sharper tries.

The big advantage of 1.e4 e5 over all other defenses to 1.e4 is that once you learn the various gambits and deviations (Scotch, Vienna, Ponziani, etc.), you have a choice among a large number of completely sound Ruy Lopez systems with Black, all of which are eminently playable, many of which are quite different from each other, and all of which give you chances to play for a win.  This is why many of the top players use 1...e5 as their main defense to 1.e4.  It is a great opening to choose for the long term, because if you get bored with one Ruy Lopez system you can just learn another.