What
@MadeOfMetal said. Everybody starts from any character till we get better. In my case, I started as GM Sub Zero, and then I moved to Alien Tarkatan, and now to Thunder God Raiden.
You need to practice all the combos shown in YT, then getting used to them all. Try this for the combos: Success any combo 10/10. If you reach 10 out of 10, then you're ready to move to the next combo. A good practice will help you out at getting used with your own character. The practice helps to learn your own character's weaknesses and advantages. This whole practice is the Knowledge.
Then comes the Experience, therefore you need to get used to your character you learned into the MUs (Match Ups) against other characters. Note that some of the characters may be a bad MU for your own character, i.e. Alien Acidic is a bad MU for GM Sub Zero. The Experience there gives you some lessons that only you need to learn and figure out some of the strategies and tactics, like zoning, corner traps, etc.
You need to learn at least 2 characters to be really that good. If you use only one, they will know who you main, and then they will pick the certain bad MUs to wreck you.
Good luck.