Pronouncing English well is quite difficult, especially for Spanish speakers. Sometimes the spelling of words has nothing to do with how they sound. There is a famous joke that says that the word "fish" could actually be written like "ghoti". Here's how:
So, yes. Pronouncing English can be hard. Fortunately there are lots of websites these days that can be of help. Here are some of my favourites:
http://howjsay.com is a nice and easy to use pronounciation dictionary.
http://www.forvo.com/ is great for all kinds of words, but especially for proper nouns like names of surames, cities or words that aren't usually in the dictionary.
http://www.shiporsheep.com/ is great for contrasting pairs of phonemes (sounds), which will help you notice the difference between different sounds. This will definitely make you pronounce things better.
BBC Learning English has a great section for pronunciation. It has got videos, quizzes and radio programmes, so it is very useful and complete.
But the most important thing is to listen to a lot of English and never be afraid to make a mistake!