If what you have is an original wwII katana, do NOT attempt to remove any rust or fix the blade yourself in any way.
The first thing to do is contact a collector or appraiser in order to verify it's value. Not all old swords are worth anything.
If it is of some value the next thing to do is seek out someone that specializes in sword restoration. If you take it to any old knife shop you run the risk of them taking a grinder to it or something just as bad.
Do yourself a favor and take the time to take care of it properly and spend the cash to have it done professionally. Cheap solutions tend to produce cheap results.