Well you gotta learn most player's 20+ % combos. IIRC one of those chicks has a 37% combo which is ridiculous imo.
Easist thing is use sub zero or scorpion/ninja smoke and use your stunning move (freeze with sz and "get over here" with the other two) to start your combos to set up. SZ has a 26% combo while the other two have a 28%. They are the easiest characters to use imo.
For a good website about all things in UMK go to www.mksecrets.net
And go on the UMK tab and it has everything you need.
Also make sure you are confident with the single player first.