On the smart car, the brakes discs are 9mm thick when new. The maximum wear they can
have before being replaced is only 1mm. You can measure them with a caliper gauge,
if they are anywhere near 8mm or below you should change them ASAP.
Brake pad minimum is 3mm. This is just the thickness of the friction material, not the metal back plate.
Once you get to 2.5mm thickness, the acoustic wear indicators start touching the brake disc.
The noise isn't something that you can ignore, it's terrible.
First of all loosen front wheel nuts, jack up the car and place the axle stand securely.
Lower onto the stand. Check for security of car on stand - you're going to be working on it!

With the wheel removed it will look like this.