Here's a few pointers that nobody's mentioned:
Know your regular attacks! Though they may not be as flashy, regular attacks are just as important as Super Moves. It’s crucial to know which attacks/Kombos hit High, Medium, and Low, and which ones pop up the opponent for a juggle opportunity. The close-up fights are like a game of rock, paper, scissors: if the opponent blocks High, attack Low; if they block Low, attack Medium. Try to anticipate what the opponent is going to do, or set them up to react a certain way. For example, start with a Low attack like a sweep, getting your opponent to block low, then attack with a Medium. Each attack could potentially lead to a Kombo, pop-up, and a juggle. Mileena’s Splits move (Up + 4) and Scorpion’s Blade Overhead (Back + 2) are both Medium attacks that pop the opponent up, which can lead to juggle Kombos.
Add to that, the fact that you can start a regular Kombo and end it with a Super Move - sometimes even after just the first hit, and you can do some cool combinations. For example, playing with Scorpion, you can start his Grievous Kombo (Forward + 4, 3) which hits Low then interrupt it with a Spear (so you would do Forward + 4, Back, Back + 1) and you can essentially Spear an opponent that’s blocking High.
Jax and Kung Lao can do similar things because they have Kombos that start low. Other have Kombos that hit Medium or Low in the middle of the sequence. Spend time in Practice mode and learn the abilities of your character.
Definitely pick one or two to focus on. I'd recommend starting with Scorpion or Kung Lao, as they are not hard to learn and are definitely some of the better characters in the game (I'd say Kung Lao is in the top 5 easily).
Hope this helps.