Hi there,
there is an easy way to do this without having to select/deleting the text in the textfield
it is called ghost text, it is simply a display pattern for fields when they equal a null value..
just go in that textfield you are talking about, in the Object properties / Field tab click on the Pattern button..
once in the Pattern dialog box, make sure you are in the display tab,
then under Pattern, just write in the textbox : null{'Please choose one'}
This should do the trick
Hope this help!