Enzo,
Arrive very very very early. You won't regret this advice. You are travelling to Sussex! It gets very busy coming down - especially on Sunday when the F1 fans come to see their gods. Sunday is very busy.
There are no good routes down that avoid the bottlenecks.
It's very slow getting out of the car parks, and the behaviour can be very bad, although there is a general migration once the F1 are done after about 3:30 pm. If you can, and the weather's kind, stay late. Prize giving at the end of Sunday usually includes a glass of champagne and cake if you stay right to the end. (I've eaten too many pieces of Lord March's cakes and watched my family scoff his champagne, so I will be off for a traditional end-of-Goodwood curry.)
There isn't a lot of shelter, so unless you have a grandstand, dress for a good variety of weather. If you have a grandstand, and they are roving, use Park opposite the house - it has the best views.
If you are watching the cars going up the hill, each batch normally runs twice on Sunday. I would watch the rally cars - you get to see some real car control.
There is a very good variety of food, but at a price of course (not as expensive as LM). The champagne is tempting, but, I think more expensive than LM.
Do not miss the Cartier Style et Lux or the Cathedral Paddock - there will be gems that you will not see elsewhere. The rally stage is also noteworthy, but takes a bit of effort. The chalk makes this very different from most other special stages.
If you fancy a being a passenger in a Porsche Cayanne around a muddy field or a drive of an Audi off-roader yourself (very challenging on wet chalk with road tyres), book as early as you can.
Make time to chat in the paddock with drivers and teams - they will be around when getting ready for or just after a run up the hill - this is one of the USPS about Goodwood - as there is no real racing, there's time to ask things like why the wheel fell off the Audi R8, what it was like to win LM IN 1971, how scared is the Pikes Peak now....?
The GAS - Goodwood Action Sports is good - these are the cycles and bikes getting air. Mini may have dodgers again - these are normally free so my family just goes again and again.
Lord March likes to innovate so I bet there will be something great that's brand new.
There's a lot to see in a day - but I hope you enjoy it.
If you see me wearing my Club Arnage hat, say hi.
Aricus