Hey! Great question
Among other things, I follow the FAA-recommended “preflight information” list, colloquially known as the N.W. Kraft list. Before going on a flight, I must be familiar with the following:
Notams - both at airports of origin and destination, and any enroute NOTAMs
Weather information - which I pull from www.aviationweather.gov or skyvector.com
Known ATC delays - if there’s a gate hold in effect, or, in IF’s case, if ATC will be open when I depart/arrive
Runway lengths at airports of intended use
Alternatives, or alternate plans of action if my flight cannot be completed as planned
Fuel required - usually I plan for fuel to get to my destination + my alternate airport + 45 minutes of cruise time after
Takeoff and landing distances available at airports of intended use - especially when the runway has an extended displaced threshold at either end.
To accomplish this, I use a combination of various websites - SimBrief.com or FPLtoIF.com for my basic flight planning and fuel, SkyVector or (because I’m also a real-world pilot) ForeFlight for any NOTAMs and other relevant airport information and approach/departure charts. I also use LiveFlight to check traffic at the airports to see if there might be any known delays.
Hope this helps!