Mental illness and the Layperson's Practice I would like to return to the practice of Buddhism.  Where should I begin?
I have a serious mental illness.  It has been well treated by medication.
When I was young, before I developed this illness I would practice meditation as best I could.
But I had no mentor and so I don't know if my practice was correct.  I do not come from a Buddhist culture so I know little about the subject other than what I have been able to read.
Will my medication interfere with meditation as it is obviously mind altering?
Should I start for example with meditation on impermanence, on mindfulness, or by attempting to reduce my desire?
A while ago, i wrote this as an answer to a similar question. I suspect you are struggling with something a bit different than the poster from that previous question, but i hope that answer can shed some light regarding the mind-altering aspect.
I don't know how your particular medication is expected to work, but giving some thought on whether you'd have an easier/harder time meditating without your medication would probably give you a clue (I am not arguing for/against using medicine here).
Provided your healthcare givers aren't giving you the red light for meditation, the choice of meditation subject is likely no different than for anyone else starting up. You can never go wrong with anapanasati. If you have the time and energy, here is a good introduction text:
https://www.accesstoinsight.org/lib/authors/ariyadhamma/bl115.html