Disconnecting from domain

I have a laptop that I used for my last client that is connected to a domain. I need to disconnect from this domain and connect the machine to my local home network. I do not have the administrator's password. How can I do this?

You cannot join/leave a domain without a local administrator account, so it's simply not possible.

However, to use a local network at home, there is no need to do so. Just plugin a network cable will do the trick (and joining a wiki hotspot also don't needs administrative privileges), at most the unrecognized network will remain as "public network", which might limit visibility and maybe a few access rights, but otherwise everything else will work as normal.
Make sure to have a local (not domain) account handy, as the domain server won't be available to validate your credentials. A domain account with cached password will work too.

I am currently required to login to the domain when I begin to use the laptop. How can I stop that? Thanks.

Ask the owner to provide a local account for you to use when in home or otherwise far from the domain. Creating that account requires to be an admin, for a single time, that's why you can't do that yourself. That created account don't need to be administrator of course.

When done, when asked for credentials, provide the username in the form "<computername>\<username>" so it knows you're trying to use a local account, and it won't bother with the domain. Enter the supplied password as normal.

I no longer have access to the administrator.

or you can just wipe the computer and set it up as a new machine. it will wipe all settings if it doesn't have BIOS password or any kind of encryption. but remember after wiping machine all data will be lost. (brand new machine)