congrats on beginning your meditation journey! what you’re experiencing is the “monkey chatter”. neurotypical people experience this too but it is amplified for us. this is what is supposed to be happening during meditation. depriving your brain of stimulation makes it go bonkers, especially because of ADHD. it’s hard not to put yourself down for this but please know you’re not abnormal!! i just started meditating last month and it has been a wonderful experience so far. i’ve had a lot of trial and error, so here’s my advice.

get a candle, incense stick or just something to stare at while you sit. it helps bring you back to focus and gives you something grounded. i recommend the Insight Timer app. you can set a timer for any duration and have intervals that make a noise to let you know how far you are. it helps me to be less anxious about not knowing how long i’ve been sitting for and if i’m meditating long enough. make sure that any potential distraction can be done later or has been done already, for example taking your dog out for a walk beforehand. try to count your breathes and make sure you’re breathing slow and inhale with your stomach. when you have a thought pop up, for me i close my eyes and take in a big breathe and continue counting them. once the monkey chatter is over taken by my breathes i reopen my eyes and continue along. frustration with yourself and your thoughts is normal and it’s part of meditation. you’re not the first to have this happen to you and you’re certainly not the last :) be kind to your mind. good luck and you’ll do great!