I suggest that you research and find the place that appears to give the best deal and take your car to them to look at the tire. Or take it to any tire dealership and get a diagnosis and estimate and then shop around.
Do you rotate the tires? The place that does my car's oil changes also rotate the tires. The tires wear more evenly that way. Also, if one tire is worn badly and the rest aren't you may have a suspension or some other mechanical difficulty that needs to be repaired before putting on a new tire. Otherwise the same thing will happen.
If the tires came with the car and your car was new when you purchased it your owner's manual should tell you how long the tires should last. If the tires are after market installed you should have a paper that gives that info.
Other wise go to the dealership that sells that brand and they can look at it and tell you.
I purchase my tires at Les Schwab but I think that they are only located on the west coast. One reason that I go to Les Schwab is that they will take off a percentage of the price of a new tire if the old tire did not last the amount of time it was supposed to last. They did this for me even tho the car's alignment was out of line.
The experts say that you should buy at least 2 tires at the same time and put them both on either the front or back, depending on whether or not your car is front or back driven. The reason for 2 tires is so that the car is balanced. If you have one new tire and one old tire the new tire will wear out sooner. At least I think that is the reason. Having only 1 new tire, if it's a different kind of tire, negatively affects your ability to control the car.
I've usually chosen to buy all 4 tires at the same time because even tho one tire is more worn out, the others are still worn and close to needing to be replaced. Four tires do cost less for each one than when you buy just one tire. I get 4 for the price of 3. I also pay attention to sales.