Which medical tests must I do before flying to Qatar, and where?

You must pass a GAMCA/Wafid medical test before your visa. Book it online at wafid.com. Pay the fee, and the system picks an approved centre for you. You cannot choose the centre yourself. The test checks your chest X-ray for TB, plus blood, urine and a physical check. Your result must say FIT. If it says UNFIT you cannot travel. Results usually come in 2 to 5 days. Do the test early and honestly. Carry your passport and appointment slip on the day. Ask the centre when your result will be ready and keep the printout safe.