As most students of Spanish are told very early in their study of the language, there are essentially only five vowel sounds in Spanish. Linguists will count a couple more because those same sounds can become slightly modified by the consonants around them. This sort of detail seldom is useful pedagogically, since the modification takes place by virtue of the consonants around them without any more effort on the part of the learner than that required to say the five vowels as they are generally taught.
We have A, E, I, O and U. The letter Y is a sometimes a vowel, sometimes a consonant and often both -- but pronouncing it correctly in all these circumstances is seldom a difficulty for English speakers because we also have the same situations in English with regard to the sound of EE (as in seen). So let's concentrate on the five vowels and how to use the phrases.
Each of the five vowels has only one mode of pronunciation -- regardless of where it appears in a word. Again, a linguist may want to complicate this with interesting observations about open and closed syllables, nasalization and so forth, but I have not found them useful in teaching beginners.
The letter A is always pronounced as the A in father. If you are from North Boston, pretend you are not...
The letter E is always pronounced as the E in they, but without the upglide created by the final Y. In other words, before you feel yourself saying the EE sound as in theh-EE, stop!
The letter I is always pronounced as EE.
The letter O is always pronounced as the O in Oh, but without the upglide as in the end of the word how; don't let the lips constrict toward the end of the word Oh and you will be in good shape.
The letter U is always pronounced as in OO in moon, unless you speak certain dialects as found in the western US, particularly California's Central Valley, where the OO can trail off into an EEEW sound. Try food -- that works too.