I need some advice

Usually there are 2 major details to keep in mind when deciding when to engage. Can your team help you? And who can you target? The ideal situation, your team is close by and you can land your ult on both their mid lane and adc. However usually if you can ensure your team a kill or 2 by hitting multiple targets usually one or the other is fine. The best way to figure out engage times is to be aggressive and learn from your mistakes. It's a hard thing to teach someone without pointing out every possibility as they are playing