First thing you want to do before modifying a car is to make sure that all of your maintenance is taken care of. That means replacing all of your filters, lubricants, service items, etc. Once all that stuff is taken care of the simplest ways to gain some power are to get an intake, header and exhaust. Also with that old of a car the cat is probably clogged up a good bit so replacing it with a new high flow unit will free up some ponies.

As for tires, with the fact that you're in a really chilly climate I would reccomend getting a set of summer performance tires on some cheap wheels (nothing extravagant, like 15" wheels with 205/50-15 tires) and then mount some snow tires on your stock wheels. Otherwise if you're not looking to do some aggressive driving then you can go with all-season tires. My favorite all season tire is the Dunlop SP5000.