Here are some websites about countable and uncountable nouns:
EN ESPAÑOL:
Sustantivos contables e incontables en inglés: «A/an» & «the» (parte 1)
Sustantivos contables e incontables (parte 2): There is/are, some, any, much & many
IN ENGLISH:
http://www.ef.com/english-resources/english-grammar/countable-and-uncountable-nouns/