As other more knowledgeable members of the forum have said, best not to touch anything and get it to a professional. The only thing you could maybe try is removing some of the surface rust with a microfibre polishing cloth and some typewriter or paraffin oil as this is low risk and a part of basic maintenance. Do this for the blade but not the nakago (tang/handle).
Interesting and coincidental timing but a dealer I know is actually selling a Bishu Osafune blade dated to the year 1492 on consignment right now...
The price on it is 1,250,000 yen (About 8,000 USD) so your blade could be quite valuable after restoration and polishing.
Your blade could potentially be a hundred years older than that or more according to Ray above, and in turn could be even more valuable.