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.
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.