Ok, so it can be weak spark or lack of fuel.
Weak spark: check the voltage across the battery (motor off), should be at least 12V. If it isn’t, replace the battery or charge it or jump-start the car. If that helps start the car you have a problem either with an old battery or with the charging system that causes the battery to be flat. With the engine now running, check voltage across the battery, should be at least 13.8 V. If it is too low check the voltage regulator on the back of the alternator, often the brushes go bad (normal wear, it’s an old car). Since you say the problem gets progressively worse this would be my first line of attack.
Lack of fuel: you did check there is fuel, right? not just based on the fuel gauge as it can fail. Can you hear the main pump prime when you turn the key?