I can get into my administer account on my Ubuntu without typing in the password how do I disable it?

I have a password on my Ubuntu computer, but if I just click the arrow next to login I can get into my account. I want to disable this, because my brothers have gone on and I don't want them to. Also, I want to disable my guest session part but I also do not know how to. Help please?

Run command :

sudo gpasswd -d $USER nopasswdlogin

Next time it will ask password.

For disable guest session follow instructions explained at How do I disable the guest session?

Open System Settings

Then go to User Accounts

On top-right corner you will find Unlock button, press it and type your password to confirm your action.

From there turn off Automatic Login

For disabling guest session you will find an answer here.

Your problem will be solved.