Sound area there mate, nothing to worry about in terms of your digs. Like clockwork regarding the Metro. You will have the choice of 4 or 5 stations in and around where you are staying, the nearest one to you is 'Fix'. M2 line up to Omonia (don't worry, most of us will use this Metro stop at some point, it's just an area I'd advise against staying in), then change onto the M1 line which runs directly up to the stadium.
If you'd prefer to go direct and avoid Omonia altogether, walk up to the touristy areas of Monistaraki or Theseio where the M1 line also stops at. Any of these journeys should take 20-30 minutes door to door.