Well, first off, what do you need done?

Try this simple trigger:

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Page 1 - Trigger: Turns Elasped [0]

<>Message: "Hello Monster"
<>

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That means, on turn 0 (before the battle even starts) the Message "Hello Monster" will appear.

I'm sure you're looking to do something much more complicated than that, though, so tell me what you need done and I'll walk you through it.