Default Selected Item And Field Value

This is my form, i would like to default the select box to USA, and default
the text field to a name such as 'Sarah' and the hidden field to another
number like ??', when the user clicks on the check box, otherwise the
fields should be blank, the value of the select box dosnt matter much, only
that it defaults to USA, when the user checks it.

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<form name="form1">
<input type="checkbox" name="checkbox" value="checkbox">

<select name="prefix">
<option value="off" selected>Select country
<option value="61">Australia
<option value="1">USA
<option value="58">Venezuela
</select>

<input type="text" name="name">
<input type="hidden" name="number">
</form>
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