Сейчас and теперь are synonyms and both are advebs of time in Russian. But there are some cases where you can only use one of them. Or one is more common than the other.
When you’re talking about what’s happening now, you CAN’T use тепрерь:
Я сейчас сплю. – I’m sleeping now.
Что ты сейчас делаешь? – What are you doing now?
But if something changed in life, for example, or it’s been a while, you can use the word теперь talking about present. Let’s imagine a situation where two old friends met after five years and started a conversation:
– Я не видел тебя с тех пор, как ты уволился. Чем ты сейчас занимаешься? Всё ещё работаешь преподавателем? – I haven’t seen since you left the job. What are you doing now? Are you still a teacher?
– Нет. Я теперь продаю одежду. Это мой бизнес. – I’m selling the clothes now. It’s my business.
We use теперь, because there’s a change in his life. It’s not like it used to be.
Раньше он любил джаз, а теперь любит рок. – He used to like jazz, now he likes rock.
You can also use сейчас here.
Another good example:
Я уже извинился. Теперь твоя очередь. – I have already apologised. Now it’s your turn.
When we say that it’s someone’s turn now, we always use теперь.
So, I would say, сейчас is more general. Теперь is more about particular situations.
I know it’s hard to understand the difference, but if you keep on learning and reading more stuff in Russian, sooner or later you would get it!
Good luck 🙂