This can be fairly simple, I've been there before.
Remove the stem bolt and spray penetrating oil down the hole, and spray some up through the fender bolt hole in the fork, if there is one.
Spray it generously every day for AT LEAST three days, letting it sit.
After the three or so days, screw the stem bolt partway back down into the wedge, but leave about 20mm of the bolt sticking out of the stem.
Secure the bike in a repair stand, or brace it on the ground. Then, you get a piece of wood- one of my most popular tools is a 7" long piece of a 2x4 board. Set the broad, flat side of the board on top of the bolt head and give it several sharp smacks with a mallet or hammer, until the bolt starts going down into the stem hole.
However, if the stem bolt is rusty I'd replace it first. Otherwise it may just break off inside the steer tube when you hammer on it.
That should break the wedge free. Whether or not it does, this next maneuver should get everything apart nicely.
Now, to back the stem/handlebars out of the steer tube, you're going to take the same piece of wood and, facing the front of the bike, put the board longways up under the "chin" of the stem. Then hammer on the other end of the board like its a chisel, driving the stem up and out of the tube.
The board should protect all the parts and their finish from any damage, and I've done this with multiple rusty old bikes with perfect results.
Let us know how it goes!