You only generate ultimate when you perform certain actions infight, which are:
Blocking an attack, dodging an attack, succesfully performing a light / heavy attack or healing someone who is injured and generating ultimate at the moment. Every time you do one of these you will generate 3 ultimate per second for 8 seconds.
Using skills usually doesn't generate ultimate. But there are some passives that can generate ultimate when you kill someone or when you use a certain skill.