There are people more qualified to answer your question, but they're not always around, so I'll have a go.
It would help to know what type of computer you have. If it's a Mac, no problem, you just add the Hebrew keyboard and toggle between languages with cmd+space bar.
If you're on Windows, it's more complicated. One easy way, however, is to type into Google Translate, using the keyboard function, then cut and paste. It's still not so direct and a real pain if you want to type various messages, such as chatting to someone on line.
There's also some help on http://www.hebrew.ecott.ch/#Keyboards