an alternative to what skyxsky said, you can use this online keyboard.

http://www.lexilogos.com/keyboard/hiragana.htm

Just input the romaji or, if you need a special character, click it.

Then hit the copy button and post it to whatever you want.

I find it helpful because, I hate switching settings on my laptop.

Depending on you computer (Pc or Mac)

1: Go to the control panel

2: Go to "add a language" (It's under "Clock, Language, Region")

3: Click the option "Add a language" 

4: Then you can search for the Japanese language

5: Select it and then download the language pack

6: After that it should show up in the toolbar, select the "Hiragana" option and then write the characters in Romaji and they will show up as hiragana letters