An IME (input method editor) is the built-in Japanese keyboard on your computer. You type romaji, it shows kana, and a Space press turns the kana into kanji. Nothing is final until you press Enter.
Space
convert kana to kanji; press again to see the next candidate
Enter
confirm what's underlined (press before Space to keep it as hiragana)
Esc
cancel the conversion and go back to kana
← →
shrink or grow the chunk being converted (Shift+arrows on some IMEs)
F7
turn the underlined text into katakana (F6 hiragana, F10 back to roman letters)
Win+Space / Alt+`
Windows: switch languages / toggle between English and Japanese input
かな / 英数
Mac (JIS keyboard): switch to Japanese / English. Ctrl+Space or Fn on other keyboards
How it works
Without an IME, type romaji and it becomes kana as you go: ka → か, shi or si → し, kyo → きょ. Double the consonant for a small っ (kko → っこ). For ん type nn, or a single n before another consonant. Long-vowel mark ー is -, and 。 is ..
In the Kanji mode you type the reading of the word shown. With hints off, the kana are hidden until you type them, so it doubles as a reading quiz.
A wrong key shakes the line and clears what you've typed for that kana. If you use your own Japanese input method, tick the box above and just type the text, then press Enter.