Ok, I have been a Debian Linux Sysadmin for decades. Now, moving to FreeBSD. These notes are what I have discovered in my recent treks.
There is an excellent handbook for FreeBSD located at https://www.freebsd.org/doc/handbook/. Probably the best documentation I have seen in a long time. Use it. Read it. Download it and store it where you can read it even when you are offline.
FreeBSD requires you to actually know something. ifup and ifdown? Nope. Manually change it with ifconfig.
Half of your Linux command either A) don't work or B) have different parameters.
The device naming convention is definitely different than Linux. Block Storage Devices are things like ada0, da0, etc… See Device Naming Conventions for more information.
A lot (and I mean a lot) of basic configuration is done in the file /etc/rc.conf.
Good news. If you really feel you must, you can install a bunch of Linux utilities. Me, I think I'll learn the new way, but you can do it. And, it appears Webmin works. Haven't tested it.
You can not simply su to root unless you are a member of the wheel group.
For Perl scripts, get away from the #!/usr/bin/perl convention. use #!/usr/bin/env perl. Note this is the preferred way at least since 2008 for writing portable Perl code. See the discussion at http://www.perlmonks.org/?node_id=716740 if you want to know more.
Most importantly for me, webmin, joe, zabbix and htop are all available under FreeBSD. :)
There are two ways of installing apps through the package manager.
ports is the one you hear about most, and for good reason. This compiles a copy of the package specifically for your machine, resulting in only the uses you need. However, it is not simple, and the package dependency management is pretty poor (think “rpm” on Linux).
pkg will install generic binary packages. A lot simpler to use, and much faster to install (ie, no compiling), but the resulting packages are definitely generic.