It's easier to learn the Orthodox with 2 ...e6 etc first. Like the Rut Lopez, most of the early moves are of a clear & simple logic improving players can understand at any level. The Slav complex with 2 ...c6 is certainly equally playable & more popular right now, but the logic of it is slightly more involved. I'd try 2 ...e6 first.
for Queen's Gambit if I am black should I play e6 or c6

You avoid the Queen's Gambit entirely by playing 1....Nf6. I don't believe that symmetrical first moves are a good idea for Black.

You avoid the Queen's Gambit entirely by playing 1....Nf6. I don't believe that symmetrical first moves are a good idea for Black.
That is not entirely true. There are many lines that transpose into QGD setups. d5 and c5 are the key pawn breaks for black (even after 1. .. Nf6), so it should not be surprising that similar positions arise.
That is true, but not entirely true. Indian systems can easily transpose into QGD, but it's AT BLACK'S OPTION, and Black avoids exhibiting the situational, psychological weakness of copying White's first move, which is something I would only do if I were playing for a draw from the gitgo, like, never. Black has almost 50% control over what a position will look like, and he must assert this control at the outset. There is an important difference between 1....c6 (Slav) and 1...e6 (Orthodox), but it pales in comparison to the imperative of asserting control at move one.
tell me