Here is a list for some of the basic things I found very useful while using Freebsd.
pkg_add -r package_name = Install a package.
The above command tries to replace the existing package. The previous is why it has -r attached to it.
If you would like to turn on verbose mode add -v to the commands, so that they will print you some output telling what is going on.
pkg_delete package_name = Delete a package.
pkg_info package_name = Display information from a package
From Linux to Freebsd (commands)
modprobe turned to kdload
lspci turned to pciconf
strance turmed to struss
Building from source with Freebsd
During the installation you probably selected to include ports collection. With ports you can build your own packages the way you like it – directly from the source. Here is a basic listing how to do it.
If you want to customize ports to your needs then the good way to start that is to check out the output from man ports command, which will give you details how to proceed even further.
If you did not install Ports collection during installation they I suggest you take a look at portsnap, which is a good utility for fetching and updating ports.