There are many ways to secure your account.
1) Do not share your password.
Basically, sharing accounts with others is not the recommended method.
Password sharing is dangerous because it is difficult to compensate for the damage that can occur after sharing the password.
2) Steam Guard installation
Mobile Authenticator needs to install a dedicated app on your phone.
[Download from Google Play]
https://play.google.com/store/apps/details?id=com.valvesoftware.android.steam.community
[Download from App Store]
http://itunes.apple.com/us/app/steam-mobile/id495369748
[Download from Windows Store]
https://www.microsoft.com/en-us/store/apps/steam-official/9nblggh4x7gm