How to prevent computer from automatically sleeping and/or hibernating?

Here are the two common Gnome 3 ways which usually disable suspend; otherwise you can try booting with the acpi=off parameter:

1. Power Settings:

   

   

2. gnome-tweak-tool (install if necessary):

   

From System Settings or by searching in Dash, go to Power settings. You'll find what you need there ;)

And to prevent from locking, go to Brightness and Lock:

Why not check out Caffeine application indicator. It does precisely what you want, and is very easy to use.