There are five main vowels in Japanese:
A: ah as in "father"
I: ee as in "eat"
U: oo as in "boot", but without puckering your lips
E: eh as in "pet"
O: oh as in "toe", but without puckering your lips
Those vowels, in conjunction with other consonants or by themselves, form syllables. Japanese syllables are said in equal lengths and tone, and are generally shorter compared to English vowels. Also, mouth movements should be restrained. Avoid moving the corners of your mouth as much as possible.
Vowels with macrons and two of the same vowels (Aa, Ii, Ū, Ō) create the same sound but are spoken at twice the length. Ei is the equivalent of Ee (for example, as in "sensei" ("teacher"/"master"). The vowel combination ai is similar to the long i as in "idol". Ee is pronounced eh - eh, not ee as "eat", and Oo is pronounced oh - oh, not oo as in "boot".
Consonants are written phonetically with few exceptions. Tsu sounds like a cross between the sounds ch- and sue, but without puckering your lips. Fu is pronounced like "who", without puckering, and with the front teeth close to the lower lip. The R sound is pronounced similarly to an L sound, but without much force, and with the tongue flicking, not rolling, against the roof of your mouth instead of your teeth.
Compound consonants such as ky-, ny-, my-, ry-, etc., are not individual syllables, as Y is not a vowel. For example, kyu should be pronounced like cu in "cute", not "kee-yoo" or "kai-yoo".
The syllables shi, ku, bu, pu, and su may be pronounced with de-emphasis of the vowel, especially when they are the last syllable of a word. For example, dōshita ("what's wrong?") may be pronounced doh - SH - ta.
The special consonant n/m is pronounced like ng in "song". For example, konbanwa ("good evening") should be pronounced kohn - bahn - wa.
Double consonants often signify an extremely short silence (called a glottal stop), a pause between the previous syllable and the next. For example, the word gakkō ("school") should be pronounced gah - [pause] - koh.