Standard demographic questions, worded respectfully and each with a "Prefer not to say" option. Only ask the ones you'll actually use to compare groups — every extra personal question makes people more likely to stop.
We've ticked a short starter set. Change it to suit you.
Age and gender
Location
Education and work
Household
5 questions selected
Tips for a better demographic survey
Put demographic questions at the end. Asked first, they feel intrusive and people drop out.
Say why you're asking, e.g. "so we can check we're hearing from everyone".
For ethnicity, use your country's census categories so you can compare with the population.