When do I start up grading my ground staff?
As early as possible.
I know that now, but didn’t do so at the start. Do I need a certain amount of W’s to buy the first upgrade.
That should be the first thing to do when you start a new airport. I recommend upgrading the services to lvl 3 then grind for more services then upgrade them to max lvl later.
Start by upgrading passenger boarding (“bus”), then cleaning. Upgrade baggage if you have planes that are still loading baggage after the passengers have finished boarding. Buy each of these upgrades as soon as possible, until you’re reliably turning planes around in bonus time.
You don’t really need to buy or upgrade fuel services very much until you start to handle M planes … you should be able to fuel 3-4 small stands with a single truck without hurting your handling time. Upgrade catering only when it’s taking longer than cleaning.
Lavatory and water should be your lowest priority, I usually don’t need to upgrade them past level 2/3 until I’ve built large stands.
Also—and others might disagree—I don’t buy any planes until my services are fully upgraded.