Introduction to Pinyin
Pinyin is a way to use Roman Alphabet to represent the pronunciation of Chinese characters. It is a tool in learning Chinese/Mandarin/Hanyu.
Pinyin has consisted of 23 initials, and 6 finals (vowels). Each Chinese character has only one syllable that contains two parts: an initial and a final.
Example:

Note: *y and w are used as "i" and "u" if they appear at the beginning of a syllable as "ya" (duck) and "wa" (doll).
Finals (Vowels)

Compound Finals
The finals with 2 or more Pinyin alphabets formed together are called Compound Finals. Please keep practice as often as you can to be familiar with them:

Tones
Chinese/Mandarin has four tones: 1st, 2nd, 3rd, and 4th. The four tones just like music in Chinese language that makes each sound level very distinguishable and expressive.

Note: There is also a neutral/soft tone. It is simply short and unaccented. Its sound relies on natural extension fo the preceding tone; and a neutral tone is also used at the end of a sentences in questions (no tone signs) as shown below:

If you would like to have a good pronunciation, it is really depending on you, in getting the four levels of tones pronounced correctly. Well, practice, practice, and practice. I believe that practice makes you perfect.