Spain´s main language is the principle tongue in Valencia. It is advisable to learn what you can before you arrive as, although more Valencians speak English than English people speak Spanish, few have enough English to hold a proper conversation and you certainly cannot rely on them to get you by.
The locals will really appreciate you making that little extra effort and having a grasp of the language will help you to integrate into society. There are many language schools in Valencia offering comprehensive classes for everyone from beginners right through to proficient speakers and the fees are fairly reasonable. Once you have learnt the basics you should have little difficulty in picking up the rest through simple day to day living.
To add to the twist of living in Valencia, the city also has its own language, Valencià, a dialect of the Catalan spoken in Catalunya. There is no need to learn this one before you arrive or even when you are here as there are very few Valencià speakers who are unable to speak Castellano although it is interesting to see the similarities between Valencià, Castillian and French. It will be useful (and interesting) to be able to recognize words on signs and in shops, just to make life a little easier for you.
So our advice at Expat Valencia is to dive in, learn Spanish and embrace the friendly locals who will really like you for having Spanish and love you for a bit of Valencia!