i think most computers have an option where you can change the keyboard.
on macs you can easily switch between keyboards, even if you dont have kanji on your keyboard you can type romaji which is automatically changed to hiragana or katakana (depends on what you pick) and after you type you can press the down arrow and see a list of options for kanji and the like
most computers made for english speakers should have some sort of function like this for lots of different languages, i'd just google and see how to set yours up
google translate is good for typing roman characters and changing it to the kana or kanji you want. you can write in that and then copy and paste. i dont know how good it's translations are, though