Rowing technique is complicated and will require many years of training to truly master.
Here are a few pointers to get you off on the right foot:
Correct technique:
Bad sequencing:
Crooked arms:
Opening your back too early in the catch:
Overreaching:
Other videos:
https://www.youtube.com/watch?v=zQ82RYIFLN8
https://www.youtube.com/watch?v=g4jiOSEigN8