when/how to say I love you

So I'm 17, my boyfriend and I have been dating for four months now and we haven't said I love you yet. I know the feeling is mutual, so I want to say it. But I'm nervous about how he's gonna react. Even if he says it back, it's a big moment and it's probably gonna take him by surprise and I'm just nervous about when and how to. I keep almost saying it and stopping myself. Can you share your first I love yous stories and is it normal for me to be worried about this?