Well, there are a couple of clues to help you get started...
"rusted pretty good"
indicates it's probably not a stainless type
"Old knife I found in a barn"
indicates it probably belongs to the same time era as the barn. I assume that's in the 100+ year catagory.
While by all means not certain, you likely have a simple carbon steel knife, or something so close to it as not to make a difference with the level of tech you're working with.
Heat treating "mystery steel" is a bit of a hit or miss operation. You could try the spark test method of figuring what all may be in it. Try a search on those words, "spark test" and you'll get plenty of info there.
Hardening isn't that difficult. Heat the blade slowly and evenly as possible, and try to do it in a relatively dark place. I say that for two reasons. One, you can more easily see the difference in where the blade is hotter when you're in a slightly darker environment, helping you avoid overheating the tim or edge (common moistakes). Second, you'll be more able to identify the tell tale "shadow line" where the steel is transitioning from one phase to another. Watch as the steel heats and starts to glow. Sonner or later, as the glow gets brighter, you'll notice a "line" of dark between two bright areas. Most typically this line will begin as a dark spot on the point, and begin moving towards the middle. This dark area is actually hotter than the brighter area in front of it. It has crossed the critical temperature, and the steel is starting to go into soloution (austenizing).
Once this shadow line has dissappeared, you're ready to quench. If you use water, heat it up prior to quenching, and shake the blade point to butt pretty agrressively to try and keep the steam from sticking to one spot, which can be a real pain, as it could crack your blade as it hardens.
Alternatively you could oil quench. With the hand tools and source of steel you're using, I assume you don't have a commercial quenching oil. I have no idea what else would be actually good for this, but I have heard of people using old motor oil, ATF, Glycol (antifreeze), brake fluid, peanut oil, canola oil, and various concoctions that use one or more of the above. Personally, I would reccomend getting at least a gallon of Parks #50 from Darren and Karen Ellis. It's not very expensive, and does the job right, with MUCH less chance of distortion and cracking.
You coul try and interrupt the quench, but I would reccomend seeing that properly done before trying it.
Then it comes time to temper your freshly hardened blade. BEFORE you temper, be sure you get the blade to room temperature, so it can FINISH hardening.
Heat up your / your wife's / mom's / dad's / whoever's kitchen oven. 400 degrees F should be a good starting point. Put the knife on the center rack once the oven's preheated, and leave it there for two hours.
If you have a good thermometer you can check the oven temperature with, DO SO! that way you are more sure of exactly what is going on.
After the two hours, use a potholder to remove the very hot knife and drop it into the sink and pur cold water on it. Now that it's tempered it shouldn't crack from this simple quick cooling.
Then, once it's cool to the touch, put it BACK in the oven for another two hours!
This second temper will help stress relieve any austenite that may have been retained from the initial quench and then subsequently transformed into martensite in the first temper.
After the second temper, you're pretty much done heat treating that blade!
If you have a way of telling how hard (more precisely than the "skate a file" trick... it's usefult to tell if it hardened, but that's about it) it is, you could then push tempering tepmerature up, or start lower, so you could dial in the hardness you're after. As a rule of thumb (not exactly correct, and there are exceptions) the hogher the tempering temperature, the softer the final outcome, but for the first few humdred degrees (up to about 400 for what I think you're probably dealing with) what you lose in hardness you more than make up for in toughness.
