When your computer is prepared for Chinese characters you will be able to write Chinese characters using the input method of your choice.
Since most Mandarin students learn Pinyin Romanization, this is also the most common input method.
When more than one language is installed on your Windows computer, the language bar will appear - usually at the bottom of your screen.
Your default language input will be shown when you first boot the computer. In the illustration below, the default language is English (EN).