You'll need a obd2 adaptor first. If you use ios then wifi, if android then bluetooth. Suggest a search for 'ELM 327' on ebay. They are no more than £15. Make sure you get one from within the uk as it'll take weeks to arrive otherwise!
Once you have this you plug it into the car (panel by drivers right knee). And pair your phone with it.
Suggest only getting the 'lite' version (free) until you have worked out the connectivity.