I often hear these mistakes made by German speakers who are learning English.
They say: I bought me a new car.
There are a couple of things wrong with this.
Firstly, remember in English we don’t use the reflexive form as much as in German. The ‘me’ is wrong. (You could replace the ‘me’ with ‘myself’ but it’s not necessary and native speakers wouldn’t always use it in this example.)
Secondly, if we want to use the past tense we need to say ‘when’ something happened.
So, the correct sentence is:
I bought (myself) a new car yesterday / last week / last month / last year.
If the ‘when’ is not important and the main focus is the new car you would say:
I have bought (myself) a new car.
Have fun learning English!
Louise