Mindfulness.
I started with just sitting down a pillow, knees confortably below my hips, eyes closed, breathing in through the nose, out through the mouth (though I just used my nose after a while).
Focused on the feeling of my chest calmly moving as I breathed. Every thought that crossed my mind was allowed, however I would not be allowed to develop it. I would just refocus on my breathing.

Some sessions would be only thoughts, some would be alright. Didn't matter, I just had to do it every day. I even started a decoy meditation habit at one point when I was overly stressed : I would start meditating for 30s, then stop, write down every "important" thought I just had to empty my head, and then start the real meditation.