Buying tires is easy, yet hard because there is so much to remember.
First, figure out which one is the best, and save it for an emergency spare. Rims are cheap, and you can store this in your garage. Gets you through a blowout and gives you time to shop around for a replacement.
Now find the worst. Look for possible worn out suspension components if really bad. If only buying two, pick the two worst for turn in.
Look for a M/S on the new tire, this indicates an all season.
Look for an A traction, A temperature on the sidewall of the new tire. "B" temp is OK, but not the best, c is gov't minimum...
Divide the price by the treadwear to get cost per mile, lower number is better.
Now break the date code, Since 2000, the week and year the tire was produced has been provided by the last four digits of the Tire Identification Number with the 2 digits being used to identify the week immediately preceding the 2 digits used to identify the year. Tires should be 9 months old or less, and not greater than 18 months.
New
Watch out for "Eco" or "Low Rolling Resistance" tires. they can wear faster, and have less traction. They can also look nearly exactly like their "regular" brother / sister tires.
Name brand are nice, but a blemished or generic with the right markings can be had for half or less of the name brand.
Any other questions?