If you are getting this game now and late, you will find yourself able to find ranked matches very easily, however 90% of the people playing will be experienced hardcore players looking for people going for achievements to beat up on. I'm sure it can be very frustrating for inexperienced players.
However, to pull off cheap victories, I would suggest mastering Kabal, Smoke/Human Smoke (hold x,y,block,run,and back before match), or NightWolf.
Practice and patience is the only way you will get this. You can of course boost now, but it is still somewhat annoying to do since they are often a lot of people playing.
A moves guide can be found here, it is 99% percent correct for every character in the game, unlike many other guides which are inaccurate.http://www.mortalkombatonline.com/content/games/umk3/
Practice against a friend or another control before jumping right into action. You will get owned before you own, there are no exceptions here. Good Luck