Look for some idioms in your mother tounge, for example... do you believe they can be understood if you translate them literally? Which message would be lost? This article analyzes why Spaniards "shit in the sea", and the following one shows us the corresponding idioms in English and Spanish.
The entry "las mejores peores traducciones del español al inglés" is a must-read, too. Yet undoubtedly, my definite favourite translators of Spanish idioms are "Súper Británico" and the creators of "From Lost to the River".
In order to learn idioms and metaphorical expressions, it is sometimes useful to visualize the literal image, which might be hilarious! and, therefore, useful in order to remember. Check this article with some English expressions illustrated.
And finally... why don´t you try to learn a few of the top 100 idioms in English?