Best mode for getting fast XP? The best amount of XP you can get from competitive games but till rank 3, deathmatch is one of the best.
Game drops for watching ESL Cologne. They didn't lied about that. Drops are real but it is really hard to get one.
How to start? Play deathmatch a lot. It will help you at start. Then I recommend you to play tournament mode. Try to play with some friends also, it is more fun and you learn faster.
What is better M4A1-S or M4A4? It depends! I like M4A1-S more because I can shoot straight into enemy player without huge change in recoil.