Since I’m new with Linux how to find out if more users than me have “entered” my terminal, also I have a password, after typing sudo I cannot type my password, nothing happens, it says password, I type It- zero.
It’s hiding the input.
Type your password, press enter, and experience the magic.
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Hi and welcome to the Forum 
- Linux has many “terminals”
- You can check who is logged into your system by issuing who and/or w.
Example:
The below is normal for a single user login on KDE plus one terminal to invoke this command…$ w 15:44:43 up 3 days, 3:16, 3 users, load average: 0,20, 0,29, 0,32 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT [hidden] tty1 :0 Tue12 3days 19:13 0.65s /usr/bin/startplasma-x11 [hidden] pts/0 :0 Tue12 3days 0.00s 1:51 /usr/bin/kded5 [hidden] pts/1 :0 15:31 3.00s 0.12s 0.02s w
- For security reasons, like others looking over your shoulders, the password is not “echoed” back while you type it in, just type it and hit the enter key.
- Just using
sudowithout anything after it won’t do anything except print the help info.
As a new user you should be EXTREMELY-CAREFUL and if at all AVOID using sudo unless explicitly told so by trusted technicians like us on the forum 
Sudo: Gives access to EVERYTHING your CPU has access to…
…And do not give SSH remote access to strangers.
And do a backup with timeshift. Learn how to restore backups (from working system, from tty, from chroot), so you could always reverse changes.
Most newbies don’t do that and usually break things… This is OK and you will probably break your system too, but having a backup is nicer than reinstalling the system.
Timeshift will back up only your system. For user files, you need to choose some other solution. I, for example, use grsync to back up files on my external drive. It allows me to access those files separately.
Since you are messing with terminal and sudo already, backups are what you should learn and do immediately.