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You type 'wo' as opposed to 'o'. At least that's how it fuctions on my computer and phone. The former gives you を which is what you want, the latter gives you お

If you're using a Japanese version of windows, then you should have an IME of some sort. The computers at school have a small toolbar usually somewhere near the bottom of the page. It's a couple of inches long and grey. There is a button you can click that will let you change the input method (or type in straight English), usually on the left-hand side of the toolbar. There is usually a little part in color (lavender?) that you can click to switch to alpha-numeric. The keyboard can probably be configured to type either the typical qwerty keyboard, or the Japanese style, so that can probably be changed depending on which you prefer (there is probably a button on the keyboard itself somewhere to do this). Otherwise, once you get the toolbar sorted out, it works as the person before said. You type the letters (either in spelled out the roman way--sensei for せんせい, or the Japanese way in which you can type a single key for a single hiragana character) and it will automatically change to either hiragana or katakana. You hit the space bar to choose which kanji to use. Hopefully this makes sense. If I could attach screencaps of what I mean, I would.

Type In Japanese