SIDs, STARs and Airways
Review and understand the information in this document - IFR Flight Planning
This document from the FAA is provided for further detail and reference; FAA Instrument Procedures Handbook
You can also access individual sections of the Handbook from the FAA Instrument Procedures Website (Chapters 1, 2 and 3 - Departure Procedures, En Route Operations and Arrivals, respectively).
Keep in mind that regulations for Europe, Asia and other parts of the world that fall under ICAO, EurOps, local regulatory or other authority may differ from the FAA regulations.
After you file your flight plan (see the basic information, below, although in some cases such as flying on VATSIM you may need only to include you departure, destination, route, altitude and airspeed) if you are flying in a VATSIM or other live-controller session you will need to request and obtain clearance. You can review the basics of obtaining clearance along with other information about communicating with ATC at our Aviation Communication page.
You should also be familiar with the basic information needed when filing a flight plan - one way to remember this is the acronym 'VITAL PPAARTS' -
IFR (there is also DVFR as an alternative to IFR in an Air Defense Identification zone
"Tail" number - the aircraft registration number of flight identification number
Where you are leaving from - the airport of departure
Petrol - how much fuel you have on board (fuel duration in hours and minutes)
People - the total number of people on board
Altitude - cruising altitude
Airspeed - cruising airspeed (true airspeed)
Route (applicable SID, STAR, airways)
Times - Estimated departure time; Enroute time
Where you are going to stop - your arrival airport (AND - if needed, your alternate airport)
You could also use FF for Fuel and Folks instead of PP for Petrol and People but that would make the acronym VITAL .... well, you get it.