Lol I work in Hvac and duct cleaning is a service we offer. We hook up a huge engine driven vacuum outside and run big hoses into the house and Hook them up to the supply, then return with a temporary blocker in between while cleaning. To clean we put in a special air whippy tool that will bang the dust and other crap out of the ducts and it gets all sucked into the big vacuum (we named it the general)
Go with a reputable company and if there are any issues after be sure to call back the idiots who do it sometimes leave the blocker Inside and it doesn't allow the unit to work.
Is it gas or an oil furnace? I highly recommend getting the oil furnaces heat exchanger cleaned yearly with an oil tune up. Duct cleanings every 5-10 years maybe longer depending how clean you aare.