Learn and practice pinyin:
★ A thorough guide to pinyin
★ Yabla: a complete pinyin chart with audio
Quick Mandarin: pinyin tables, tests and games. The content you need is at the very top of the page.
Arch Chinese: a pinyin table with all possible tones of each syllable, and pronunciation explained for English language speakers

Pinyin convertion:
If you need to type pinyin, here are some tools which can help you convert to it:

★ with Google Translate you can achieve the same by typing characters. Simply type your text in Chinese, then click on the Ä (capital A with an umlaut) in the bottom right corner and you will see the pinyin below the box.
!Sometimes there might be mistakes in the syllables or tones, so use with caution!
Chinese Tools: here you type the syllable followed by the tone. For example, for "Wǒ huìshuō hànyǔ", type "Wo3 hui4shuo1 han4yu3".
★ On this one you can proceed in the same way. I personally type the entire text and then go back and mark the tones. You will find that if you make a mistake with the tone, all you need to do is click anywhere in the syllable and write the new one. It changes automatically.