How to deal with corrosion?

I’ve just got myself an old and expensive military knife, trouble is it’s got a fair bit of corrosion on the hilt and a little on the blade. I know how to take care of blades well but I’ve never had one so badly damaged. Does anyone have any suggestions on how I’d be able to repair it without causing more damage than it already has?
Forgot to say, the knife is steel. (Before stainless steel was used)

You better get it professionally restored, cleaning it yourself could ruin its value.