How Can I Improve My Server Security

A few days ago some stupid Neighbour or so Hacked my AP, changed the Admin Password and deleted every Port-forwarding rule in it.

I repaired the Damage and looked at the log file on my Media-Server. This Guy really only Hacked my WiFi but on the Server logs are some Alarming lines. I use an Ubuntu 14.04 Server and I changed after that every standard Port and Password.

My Questions are,

1. How can I better secure the Server against Attacks? (specially SSH )
2. Can someone tell me please what is going on in my "auth.log"

No IP address in this file is one of mine.

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Thanks for any Help!