We recommend that you get familiar with archive data and their availability using MARS browser

You can also get the request syntax using "View MARS request" feature.

verb,
 keyword1 = value1,
      ... = value2,
 keywordN = valueN

retrieve,
 class="ei",
 stream=oper,
 expver=1,
 levtype=sfc,
 param=165.128/41.128,
 step=0,
 grid=0.75/0.75,
 time=00,
 date=2013-09-01/to/2013-09-30,
 type=an,
 target="ei_sfc_165_41.grib"

#action
retrieve,

##IDENTIFICATION KEYWORDS
https://software.ecmwf.int/wiki/display/UDOC/Identification+keywords

   #Specifies the ECMWF classification given to the data
    class="ei",

   #Identifies the forecasting system used to generated the data when the same meteorological types are archived
    stream=oper,

   #Is the version of the data, 1 is OPERATIONAL
    expver=1,

   #Denotes type of level
    levtype=sfc,

   #Meteorological parameter
    param=165.128/41.128,

   #Determines the type of field to be retrieved:
    type=an,

##DATE AND TIME KEYWORDS
https://software.ecmwf.int/wiki/display/UDOC/Date+and+time+keywords

   #Specifies the forecast time step from forecast base time
    step=0,

   #Base time
    time=00,

   #Specifies the Analysis date, the Forecast base date or Observations date
    date=2013-09-01/to/2013-09-30,

##POST-PROCESSING KEYWORDS
https://software.ecmwf.int/wiki/display/UDOC/Post-processing+keywords

   #Output grid mesh
    grid=0.75/0.75,

   #Area
    area=75/-15/30/42.5 

##DATA STORAGE KEYWORDS
https://software.ecmwf.int/wiki/display/UDOC/Data+storage+keywords

   #specifies a Unix file into which data is to be written after retrieval
    target="ei_sfc_165_41.grib",

##OPTIONAL MARS SERVICE KEYWORD
   #(Optional) argument to retrieve NetCDF format
     format="netcdf",