A very simple way to do it, if you are OK with a simple message would be creating a document-level JavaScript with the following code:
app.alert("This form is fillable") ;
If you want some eye candy, a still simple way would be to create the message in the layout program, save it as PDF, and then add a button field (read-only, no action) and make that PDF the icon of that button. Then you add another button, labelled OK.
The action of the OK button is to hide the message button and the OK button itself, and then set the focus to the first field (which would best be covered by the message button field. The JavaScript code for the MouseUp event of the OK button would then read:
this.getField("theMessage").display = display.hidden ;
event.target.display = display.hidden ;
this.getField("theFirstEntryField").setFocus() ;
You would save the form with the message button and the OK button being visible. If you want to show the buttons every time the document opens, even after it has been saved by the user, you would create a document-level script:
this.getField("theMessage").display = display.visible ;
this.getField("theOKBox").display = display.visible ;
and that should do it.