This is a tip for all the questions: get ServInfo (if you haven't already) and take a look from time to time at the traffic around you.
-What do I say on Unicom if there is no ATC at the airport im departing from?
Take a look at the Servinfo screen. If there are other planes arriving/departing to/from the airport you are at (let's say EGCC), you should say something like this: "Taxiing to rwy xx EGCC", and when you reach the Holding Point of the desired runway, say "Taking off rwy xx EGCC". If there is a plane on approach at less then 10nm, it's recommended you let him land first and then take off.
What do I saw on Unicom if theres no ATC at the airport Im landing at?
Again check for other traffic and report "Inbound ILS rwy xx EGCC" or visual, or any other approach you are performing. If the rwy is clear, go ahead and land... Don't forget to report "Rwy xx EGCC vacated".
What do I do if there is no Controllers in my airspace, so should I just follow my flight plan?
Yes.
How do I know when to contact the ATC (if there are any at the airport) when im going to land?
Normally, the controller contacts you first with a private message like "This is EGCC_APP, please contact me on 119.4", or something like this. However, if the controller does not contact you should contact them like this (in my oppinion): APP at ~40nm, TWR at ~15nm, GND when you have vacated the runway.
Hope this helps, if anybody thinks otherwise, please feel free to correct.