In English we have 5 vowels orthographically(in spelling), but phonetically(in terms of speech sounds) there are 14 different vowel sounds! Spanish only has 5 vowel sounds! This can make it rather difficult for Spanish speakers to pronounce English words. For English speakers learning Spanish, this means that we have to narrow down our vowel pronunciations.
When it comes to pronunciation, I always tell my students to practice their vowel pronunciation in Spanish:
a-e-i-o-u is pronounced, in Spanish, "ah"-"ay"-"ee"-"oh"-"oo"
The good news is that these vowels never change in Spanish pronunciation. Once you get those down, you can pronounce any Spanish word with confidence.