how to modify field of a node? I have a custom text field which I use to display some voucher code to users.
I have it in a custom content type named (machine name) : 'add_deal'.

What I want to do is to style the field when the node is displayed and add a link/button next to that field to copy the text in that field.

I created a field--field-type--cotent-type.tpl.php file, but how do I use it, and I dont think its working for me either. Is there any hook function I need to use to make use of that tpl file?

How could I achieve it?
I think you have to use your field name instead of field type in the name of your .tpl.php file. Assuming the label of your field is 'Voucher code', then the machine name of this field would normally be field_voucher_code and your template file should be named field--field-voucher-code--add-deal.tpl.php See theme_field documentation for more details.

Don't forget to clear the caches (i.e., rebuild the theme registry) after creating this file in your (custom) theme.