Do you mumble?
Mumbling is how you sound when you don't speak clearly (when you don't enunciate.) Diction is the way you enunciate, articulate, pronounce and deliver spoken ideas. You already know that pronouncing a word incorrectly makes it hard for people to understand what you're saying--and can literally change the meanings of words.
In English, your breath and every part of your face (tongue, teeth, lips, jaw, gum ridge and soft palate) is used to speak clearly. Also, you need to open your mouth more widely than you do when you speak Spanish. And your voice must either go up or down when you say different syllables. In other words, speaking English clearly will exercise all the muscles of your face more than usual (which can be tiring.)
Vowel sounds in English are very important. Here are a few things you need to remember about them:
They're always included in syllables, (and every word has at least one syllable) so the pitch of your voice will either go up or down when you say them.
When a vowel is a stressed syllable, you must prolong the sound. Unstressed syllables must be said more quickly and in a lower pitch.
The 15 English vowel sounds are made up of three different types of sounds that you make with the front of your tongue, the middle of your tongue and the back of your tongue.
You must open your mouth more when you say vowels than when you say consonants. If you can't fit your finger between your teeth, or if your teeth are touching when you say vowels, you won't be able to say them correctly.
Consonants are sounds that are made when your breath meets obstacles in your mouth. They don't sound as smooth as vowels (they pop, hiss, snap, or hum.) Most (but not all) English words have at least one consonant, and some have many more. Some things to remember about consonants:
When they are "voiced," they sound louder than "unvoiced" consonants and you can feel a vibration or buzzing in your throat if you touch your throat gently when you say them.
Voiced consonants are made by:
touching or almost touching your lips together (p, b, m and w)
touching your upper teeth to your lower lip (f, v)
touching the tip of the tongue to the bottom edge of the top teeth
putting your tongue between your teeth (th)
touching or almost touching the tip of your tongue almost behind your top teeth (t, d, s, z, n, l)
touching or almost touching the blade of your tongue to the hard palate (y)
touching the back of your tongue to the soft palate (k, g, and ng)
and "h" is pronounced using the space between your vocal chords.
Although this is a lot of information, it is just the beginning. (For more information on the way to articulate different consonants, please visit http://ocw.uci.edu/upload/files/consonants.pdf )
Diction is something that can be perfected only with a lot of thought and practice--but it's definitely worth the effort.
More resources:
http://rachelsenglish.com/videos/the-voice/basics/ This is one of the best websites I've seen for learning all aspects of correct pronunciation.
https://www.youtube.com/watch?v=_M7EZ2zAjVU This is an excellent video that explains how the different positions of everything that we use to make different sounds (tongue, jaw, lips, etc.) and it gives you exercises you can use to improve your diction.
https://www.youtube.com/watch?v=uQn4OYC3etY This a good explanation of how expressive your face must be to speak clearly.
https://www.youtube.com/watch?v=mfZscA9V6nU: This website is recorded by a British speaker, but the same principles apply in American English.
https://www.youtube.com/watch?v=3LD7m3luv0Y This website gives you a chance to see what your mouth should look like when you are making the sounds of the alphabet.
