It's your first steps in D open tuning - in other words DADGAD - and you don't know where to start your learning.
First of all, you need to properly tune your guitar. Then start playing your first chords.
Why play in DADGAD
The first characteristic of this D tuning (called D open chord/tuning) is the fact that you have 3 open strings in D. This reminds of a pipe drone, a concept very dear to Celtic music in general and to Breton music in particular.
The second characteristic is to create rich harmonies. In other words, this type of tuning makes the open strings resonate with others, creating rich sounds suited to Breton or Irish music. That's why all of our teachers play DADGAD open chord.
How to tune your guitar in DADGAD
It is not always easy, once you are used to standard tuning, to change your tuning to open DADGAD.
Lower the E string so that it sounds like the D string (3rd from the top in standard tuning).
Lower the B string (5th in standard) so that it sounds an octave above the A string (the 2nd).
Lower the high E string (6th in standard) so that it sounds like 1st and 3rd.
And always remember that you have to LOWER the tuning, never go up!
10 chords to start in DADGAD