Hallo Sina
I've been there a few times, the circuit is a like a stadium, you can see most of the track from any point in the stands, there's a Montaña area (cheapest) whic is on top of a hill, but it's very far away from the track (though there's a couple giant screens there showing the race).
The circuit is a few km away from Valencia, I'd say around 15km, the closest town is Cheste, where most of the party goes on.
About accomodation I can't tell you much (since I live about 5km away of the circuit I never needed it!),but I seen there's "camping" sites around the track, in Cheste and of course in Valencia you'll find lots of places to stay. Maybe you should ask to other forumers who've been there before like Ooost and some of the British guys.
Transportation, if you're in Cheste you don't need, you're already in the middle of everything. If you're staying in Valencia there's a train stop next to the circuit, trains come from the Estación del Norte in central Valencia. By road it's an autobahn so it's quite fast, but on sunday morning it will be
jammed (beware or you'll miss the 125cc race!)
The airport is about 6 or 7km away from the circuit. I think there's now a train/subway stop there that takes you straight to Valencia centre or you can take a taxi.
Tickets are selling quite fast, so I would try to arrange that ASAP. You can see what's left and buy tickets (click on "comprar")
here