The majority of the Japanese use the QWERTY layout, and after typing a chunk of words in romanized characters, hit the space bar to get the Japanese scripts, and then enter.
※There are several versions of romanized characters in Japan, so which one you need to use depends on your computer's configuration.
Very few people use kana input, although if you master it, your typing speed will become, theoretically, twice faster because you don't need to hit two or three keys to get one Japanese letter.