We have a small guide dedicated to Valencia, which features some useful information about the city as a whole.
Additionally, we have some interviews with expats who are living in Valencia, like Billie and Rachel, who both teach English and Zach, an American expat. These can provide some insight into what life in Valencia is like, and answer your questions about meeting people and making friends. It seems that learning a bit of Spanish can definitely help with this process. There are also many Meetup and Facebook groups that can help you meet people.
We also have an article written by an expat about volunteering abroad. The website Do-it is can be useful in this regard.
We have an article dedicated to what you need to plan for prior to your move, which you might find useful in your preparations, as well as another article that covers other factors to consider.
With regards to doing CELTA, alternatively you could do a TEFL course, both are widely accepted, and the TEFL is less intensive. However the CELTA course might guarantee a better job offer. Is there a reason you have decided on doing the CELTA?
I found an article for you that explains the differences between CELTA and TEFL. Perhaps as a suggestion, you could begin the course online now, as you said you are only planning on relocating next year. However, if you are interested in doing the course when you move to Spain, here are some CELTA courses in Valencia:
Speaking from experience, I did a 120 hour TEFL course (most schools want at least 100 hours) with i-to-i, which consisted of 100 hours online work (assignments and tests) and 20 hours of practical (which was held over the weekend). In total it took me a month to complete. This enabled me to teach English in South Korea. However, when I was interested in teaching English at a language academy in South Africa, I was required to have the CELTA, as it's more extensive, and specifically for language schools.
One thing to keep in mind with Spain is that people can teach there without a degree and just a TEFL/CELTA, so having a degree gives you an advantage already.