I agree with @
BlastX21 and @
Fred Marvel ! When I picked this game up (the 1st fighter I tried to get decent at) I had the same problem your having now. I couldn't do any combos and I would input the wrong move. I eventually picked up Doomsday as he is not a character that needs to do combos to win, unlike most of the cast in this game. I'm not saying pick up Doomsday, unless you want to. But since you said you wanted to play GL, go with him 1st since he is one of the better beginner characters in this game! And a BIG TIP is to stay positive whether you win or lose!
Don't get discouraged! Your gonna get bodied! Its gonna happen. Trust me. Hell. I STILL get bodied!
Practice! Practice! Practice! Inputs, combos and all that fun stuff!
Learn from your mistakes!
What also helped me is watching Youtube channels that post Injustice! Watch Combo videos and Tournament matches!
Doing all that REALLY helped me get pretty good at this (at-least online) and I went from playing Doomsday who is hella easy and Braindead as fuck to play. But now I'm playing Lobo! Who is considered to be one of the harder characters to play in this game. When he 1st came out I couldn't do a single combo with him. But now I can do all his bnb's with my eyes closed!
With all that said however! You should mainly focus on is to just get the feel of this game. Learn one combo that does roughly 30% damage or more with whatever character you wanna play and practice it over and over. Than play some online matches.
And the most important thing in any game is to have fun! Cause..whats the point of playing the game if you don't have fun?