That's probably a constant current led power supply so the only way I think you can
"dim" it is to use a variable dc voltage regulator. You can't PWM a constant current
led supply. (it would just flicker)
Either you vary the dc input voltage or you replace the supply with completely different
circuit.
Can someone help me with which components and circuit i can build an Dimmer with an Arduino?
You will have to decide if which approach you want to take.
You can rule out any approach that involves PWM control of the 12V input to the constant current
led supply you have currently. That is not an option. You can buy a benchtop variable power supply
or borrow one from someone to experiment with lower input voltages for "dimming" the leds.
If that doesn't work for you I suppose you could return the supply and simply say you don't need it.
The question is , it this the only instance in your life where you will need to prototype an electronic
circuit. I hate to say it, but Electronics is not well suited for a "one night stand". Either you make a
commitment , or suffer the consequences of being a "weekend warrier". That's fine if you commit to
learning a little every weekend, but if you think you can just jump in , build one circuit and then give it up, I have news flash for you: Once you realize you can do it, you will come back for more and sooner or later you are going to wish you had a Benchtop variable dc power supply for prototyping. I started
out by building my own (38 years ago) . I built my first bipolar supply with foldback current limiting into a wooden cigar box. (probably not the best choice for a chassis by the way). It had fixed +5V,
+12V and variable +/-0 to 18V dc , using
LM723s
12V PWM CONTROLLED LM-317 VOLTAGE REGULATOR
The datasheet indicates the LM-317 can supply 12V dc out @ 1.5A (18W) (this would require a TO-220 HEATSINK)