Everything in <> is variable and depends on your system / your intention

The prompt is always shown in your shell: 
<user>@<host>:<path> $

<user> is the user that is running the shell (important if you work with multiple user accounts e.g. root and user)
<host> is the hostname of the PC the shell is running on (important if you connect to remote machines)
<path> is the path where you are currently located at

As soon as the prompt is shown you can enter any command.
The 'TAB' key autocompletes commands and paths as soon as there is no ambiguity.
If multiple possibilities exist 2x'TAB' will diplay them.

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Commands you should know:
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man <command>				show manual for the command <command>
							exit with 'q'

ls <options>				list directory (mind the space between command and options, this is always the case)
ls							<options> is empty		just list the directory
ls -l						<options> is '-l'		list directory with more details (permissions owner group size date time filename)
ls -a						<options> is '-a'		list directory including hidden files
ls -a -l					<options> are '-a -l'	list directory (also hidden files) with more details (mind the space again)

cd	<path>					change directory (relative to your current directory)
cd /<path>					change directory (absolute [from the file system root])

mkdir <folder>				create folder
mkdir -p <path>				create all folders to this path

rm <file>					remove the file
rm -r <folder>				remove folder (recursive)

chmod <permissions> <file>	change file permissions (read, write, execute) for owner, group and others
							Permissions are typically shown this way: rwxrwxrwx
							first entry (rwx) is for the owner of the file/folder
							second entry (rwx) is for the group this file/folder belongs to
							third entry (rwx) is for all others
							missing permissions are shown with a dash '-'
							rwxr-xr-- means the owner can do anything; the group can read and execute; all others can only read

chmod +x <file>				make file executable for everyone (+w -> writable; +r -> readable)
chmod -x <file				remove the right to execute for everyone (-w -> writable; -r -> readable)
chmod u+x <file>			make file executable for the owner (u=owner, g=group, o=others a=all)
chmod u-x <file				remove the right to execute for the owner

cat	<file>					read file contents (prints the whole file to the terminal)
less <file>					read file contents (page by page) [exit with 'q']

echo <text>					print <text> to the terminal
echo <text>	> <file>		print <text> to the file <file> (note the '>' before <file> )
							using the operator '>' the output of every program can be written to a file
							what would 'ls -l > out' do?

nano <file>                 edit a file with the "nano" CLI text editor, creates the file if not existing

./<prog>              		executes a program in the current directory
/.../<prog>           		executes a program in an absolute path (from root)

/.../<prog> &				executes a program in background
/.../<prog> &>/dev/null		executes a program silently with stdout and stderr redirected to /dev/null
/.../<prog> &>/dev/null &   executes a program silently and in background

chrt -f <prio> <prog>    	starts <prog> directly with RT priority <prio>, sets policy SCHED_FIFO
chrt -r <prio> <prog>    	starts <prog> directly with RT priority <prio>, sets policy SCHED_RR
chrt -f -p <prio> <id>		changes RT priority of running process/thread ID <id> to <prio>, sets policy SCHED_FIFO
chrt -r -p <prio> <id>		changes RT priority of running process/thread ID <id> to <prio>, sets policy SCHED_RR

sudo <cmd>                  executes <cmd> as root user

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Paths you need to know:
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/ 							the root of your file system tree
/home/<user>/				the home directory of the currently logged in user <user>. Shorthand: '~'
/sys/class/gpio/			access to the gpio pins (if available) - see sysfs