Don't ONLY focus on doing combos online, though.
Get to know your character and how your character matches up VS different characters on the roster.
Get to know your character's strengths and weaknesses.
Combined with those 2 things (match up knowledge + strengths & weaknesses), combo knowledge can go a LONG way to help you getting wins online.
Also, there's lag online, where there is NO lag playing offline.
You need to practice getting your timing online, which is different than playing offline.
Don't just expect your online combos to work exactly as they do offline, as you may have to pause between inputs differently online than you would normally offline.
The specific character threads can help you A LOT too, as people have spent time mastering those characters and getting better with those individual characters.
I hope these tips help.
Welcome to TRMK, and to MK9.
Happy gaming.