The SSH server is enabled by default on a Raspbian image. And none of the security settings are enabled by default, leaving it to accept connections.
The default login is 'pi' with password 'raspberry'. If you are unable to log in to the rpi from a remote connection with those credentials, then you are using an invalid/corrupt/hacked image. I recommend downloading it fresh from www.raspberrypi.org.
The fact that you said you are using sudo to SSH into the rpi tells me you are attempting to connect from a different unix machine. Is this true? For one, you do not need sudo to issue SSH commands as a regular user. And second, where are you attempting to connect FROM?