Spoilers: (Highlight to reveal)
I highly recommend you follow the following mindset, as it will help you survive the online world of Mortal Kombat, and majority of fighting games for that matter:
-Nothing is cheap.
-Everything is cheap.
-There is no such thing as spamming.
-Do not hate any character simply because you are not experienced with them.
-Do not hate any PLAYERS who uses characters you are not experienced with.
-There is no such thing a 'the best character'. You use who you are comfortable with, or who you want to master.
-There is a strategy around everything. Don't doubt so easily.
-Do not expect nerfs or buffs to save you. If you can survive without those nerfs or buffs, isn't that already proving yourself better?

-Losing does not make you a bad player.
-Contributions to losing can include:
a) Inexperience with the character you are playing.
b) Inexperience with the character you are fighting.
c) Inexperiences with the strategies being used against you.
d) Your general mood, emotion, and energy effecting your performance.
e) Lag.
f) You were of equal skill, but your opponent got the upper hand OR you had a small brain fart, which happens to everyone.
-Problems (a), (b), and (c) can be solved through practice, experience, and asking for help from others in terms of tips.
-If you are experiencing problem (d), turn off the game and do something else.
-If you are experiencing problem (e), ......sorry.
-If you are not experiencing any of the other problems, you are experiencing problem (f).
-Please take absolute note that problem (f) IS NOT A BAD THING. Problem (f) WILL HAPPEN and is the best way to lose, as it means you lost a completely fair fight that you were ready for.
-Again, losing is not a bad thing.

-If you rage quit, you are completely giving your opponent a message saying, "I'm sorry. I cannot handle defeat. Let me leave now, as I cannot emotionally handle the thought of losing." If that's okay with you, then by all means.
-If you must step away from the controller for reasons of random possibility, set the controller down and accept defeat. Life is more important than this game. Again, losing isn't a bad thing.
-Do not give hate mail, as it is the same as saying the same message given for 'rage quitting'.
-Do not RESPOND to hate mail or rage quitting.
-If someone gives you a 'gg' message, reply with a positive response...no matter what negative remarks they might give. If it is truly negative, it's better not to respond at all.