If your English is breddy good, Duolingo. If you want Spanish, try http://es.lernu.net/index.php
>I'm surprised to understand some stuff. Is it because I'm Spaniard?
Yes…Much of Esperanto root vocabulary comes from Latin and French, so you will find many similar words to Spanish. For example I'll try to think of sentences using mainly Latin roots:
La akvo estas en la taso. (La agua esta en la taza.)
Mi duŝas min en la ban-ĉambro. (Me ducho en el cuarto de baño.)
Pardonon–mi ne bone parolas la hispanan!