Q:

How to export streets in a certain area from one shapefile to a new shapefile?

I have a shapefile with the roads from Europe and I want to export in a new shapefile only the roads from Greece. 
I tried to delete the other roads but there are too many and I have no attribute to help as an example country.
the attribute is 
  gid serial NOT NULL,
  sourceid character varying(50),
  exs numeric,
  notes character varying(254),
  roadid character varying(50),
  onme character varying(200),
  rtenme character varying(200),
  ntlclass character varying(200),
  fclass numeric,
  crgway numeric,
  numlanes numeric,
  lnewidthm numeric,
  rdwidthm numeric,
  axleloadmt numeric,
  totloadmt numeric,
  srftpe numeric,
  srfcond numeric,
  srfprep numeric,
  isseasonal numeric,
  curntprac numeric,
  gdwthrprac numeric,
  bdwthrprac numeric,
  speedlimit numeric,
  curntspeed numeric,
  gnralspeed numeric,
  isundrcstr numeric,
  cstwrketc date,
  graddeg numeric,
  sec numeric,
  hasshouldr numeric,
  hassidewlk numeric,
  drivside numeric,
  iselevated numeric,
  hasmedian numeric,
  opstatus numeric,
  length_km numeric,
  shape_leng numeric,
  geom geometry(MultiLineString,4326),

I use postgres and now start to use QGIS 

A:

If you don't have an attribute, you should first find a polygon feature class with the boundaries of Greece (e.g. on gadm.org).
Then you have two solutions : 
You want the roads to be cut at the boundaries of you area of interest : use some clip tools (in QGIS : Vector -> Geoprocessing -> Clip )
You want to keep the two sides of a road when it crosses a boundary. Use a selection by attribute, then export the results. (in QGIS : Vector->Research tools->Select by Location, then Save As...)
A compromise is to create a buffer (Vector -> Geoprocessing -> Buffer) around Greece and clip with the buffer, so that you keep the network connectivities but you don't have irregular pieces of roads outside the country.

A:

You can also use a polygon to select the roads you want (in the top menu bar, the icon with the white arrow and the dotted lines). Then click on the layer containing the roads >> save as shapefile. If you make sure the box 'only save selected feautures' is thicked, you're new shapefile should only contain the selected roads.

