Not so much a where to more a where not to.
We went to valencia and stayed at the track initially we thought there would be a campsite.
It is a something alright, the campsite at the track was a large overflow carpark and being a rocky terrain thats what you slept on, rocks. No facilities,toilets or showers, not normally a problem for us blokes but the girlies might object.
There are plenty of cheap and reasonably decent hotels and taxi's are cheap.
For the track, look at the track map then aim at the top i.e. 12 o'clock area. the track is mostly banked spectator areas and so some views are good some are better. Turn 2 is best for viewing wobbly momments but from position 12 o clock you can see almost all of the track. using the clock the start line would be at 6 o clock
It's a great atmosphere and sunshine. supa lovely