Vowels in Japanese
a is pronounced like the "a" in "star".
i is pronounced like the "e" in "we".
u is pronounced like the "oo" in "boot". This vocal is pronounced with the lips compresed.
e is pronounced like the "e" in "get".
o is pronounced like the "o" in "oh".
The Japanese vowels have different lenght, long and short, during the pronunciation in some words. You have to be careful about this because the meaning of a word can change if you don't pronounce it correctly.
For example: if you don't pronounce correctly onii (older brother), you are pronunciating oni (demon).
In addition you have to speak as clearly as possible when you're talking in Japanese because there are many word that have a similar pronunciation like byouin (hospital) and biyouin (beauty salon). For this reason you have to learn the pronunciation and practice it for talk Japanese.
Japanese Vowels videos