How to create a startup pop-up window

Hi, i'm an administrator of a rental house and i want to add a pop-up window ad the startup of the OS of our MacBook and MacPro, so our client can accept our term and condition and just click "OK".

Does someone have any idea how to do this ?
Thanks!

You would create an app that does this, and have it set to start at login.

I think we could give you better ideas for a solution if we knew more about the parameters. How much text is going to be displayed? What is the purpose of the message? Is it displaying some type of Terms of Service agreement where you need to track whether the user has agreed to the terms.

A couple of things,...

Are these computers public access computers? Or common area computers?

If so I would consider enabling the Guest account with Simple Finder enabled and employing Parental Controls.

You could then enable the Lock Message under the Security and Privacy System Preference which will appear on the login and lock screens and use this as the T&C prior to login. Guest does not require a password but the T&C would state that logon to the Mac assume agreement to T&C.

There are also management capabilities by using OS X server to install "Profiles" just like iOS deployments. Login banner messages among other features can be configured for managing Macs.