Don’t worry! Finding a student flat in Valencia is quite easy: Valencia, in fact, is essentialy a university city, and each year is home of many Erasmus students (is the second city of Spain, after Granada, for number of Eramus). The first suggestion is to arrive soon in the city, when there is more choice, and to look for the flat directly when you will be there.
To find the flat of your dreams there are more than one option. Websites like Loquo, Easypiso, Idealista or Enalquiler publish ads in real time, and also by walking in the streets around the University you will certainly find a lot of ads attached on the light poles and on the walls of the faculties. Equip your self with a spanish mobile phone and call the numbers which you are interested in, probably they will give you an appointment within one hour. The agencies can also be a solution, but, obviously, they require a quite expensive commission. In your hostel you will meet many erasmus students in the same condition of you: looking togheter for a flat will alleviate the troubles of the research and will be the first opportunity to make new friends!
Choose carefully the area where you are going to live in: living away from center or from University may be cheaper, but maybe can be a problem for moving, even though public transports are good. Anyway, the rent is usually between 150 and 300 euros, never higher.
Here, you can find an useful vocabulary for your search.