There are 2 ways, the easiest is from the console (the screen you see when you boot).
The other way is using a method called ssh.
The first way, at the console, you'll see a 'list' black & white text with choices 1-11, like seen just below. Choose #9 and [Enter] and you'll see the shell prompt which looks like:
% <--This is where you'll type your commands.
The other method requires a client program, which depends on whether you're running Windows/MacOS/Linux. See this link for a comparison of client programs you can use:
Comparison_of_SSH_clients
For Windows a program called Putty is pretty popular, it's here:
Putty
For Mac, I'm not sure what is preferred, but there's this one:
Rbrowser
For Linux, from the command line/terminal you would just type:
ssh -l admin IP-address for example 'ssh -l admin 192.168.1.100'
and enter the password which is 'freenas' unless you've changed it. (You should)
Once you install one of the client programs, you just need to look for the place to enter the IP address of your FreeNAS system, enter the username 'admin', and tell it to connect. After it successfully connects, you should also see the % prompt.
If you're using ssh, you may need to gain root privileges by typing 'su' to run some commands. When you type 'su' and press enter, you'll need to know the root password which you can set from the GUI by going to Account -> Users -> View All Users and looking for 'root' and clicking on the button 'Change Password'. The default password for both 'admin' and 'root' is 'freenas'.
Please give me feedback in case I missed something so I can update this.
The other way is using a method called ssh.
The first way, at the console, you'll see a 'list' black & white text with choices 1-11, like seen just below. Choose #9 and [Enter] and you'll see the shell prompt which looks like:
% <--This is where you'll type your commands.

The other method requires a client program, which depends on whether you're running Windows/MacOS/Linux. See this link for a comparison of client programs you can use:
Comparison_of_SSH_clients
For Windows a program called Putty is pretty popular, it's here:
Putty
For Mac, I'm not sure what is preferred, but there's this one:
Rbrowser
For Linux, from the command line/terminal you would just type:
ssh -l admin IP-address for example 'ssh -l admin 192.168.1.100'
and enter the password which is 'freenas' unless you've changed it. (You should)
Once you install one of the client programs, you just need to look for the place to enter the IP address of your FreeNAS system, enter the username 'admin', and tell it to connect. After it successfully connects, you should also see the % prompt.
If you're using ssh, you may need to gain root privileges by typing 'su' to run some commands. When you type 'su' and press enter, you'll need to know the root password which you can set from the GUI by going to Account -> Users -> View All Users and looking for 'root' and clicking on the button 'Change Password'. The default password for both 'admin' and 'root' is 'freenas'.
Please give me feedback in case I missed something so I can update this.