Version: Odoo 8.0

I'm using Odoo to centralize internal purchasing for the company so that whoever needs supplies or tools can just select what he wants from a pre-made list and ask the central purchase office to buy it for them.

So far I'm going good except that I can't figure out the following:

I'm creating several users with different group policies.

Administrator: can actually modify the software structure and change logons

Central Purchasing: get all requests, load items, change prizes and manage all the database

Office Heads: those have to approve whatever the User want to buy so that there's no risk of buying useless things or waste time for orders that were already made

Users: they can view items and order

My question is: How can I change the submit button that send the order to the Central Purchasing going via the Heads first and then add to the Heads interfaces a checkbox to confirm the order and finally submit it to the Purchasing?

I'm new in the software so I don't know how to actually modify the front-end and write functions in the back-end.

Thanks in advance