You said you wanted the cheapest way; as others have suggested I think the cheapest way possible is to get a second-hand Tacx Flow Smart trainer and a cheap second-hand bike with a road tyre at the rear. That’s what I did, I fell in love with Zwift and then upgraded to a Kickr Core after a year.
I’m pretty sure the Tacx Flow was the cheapest trainer ever made which changes resistance and simulates the hills you get on Zwift automatically. I bought mine second-hand for £110 in 2019 and they’re now available for as little as £50. You could find a Tacx Vortex for a similar price; just make sure you don’t get a Blue one, as that doesn’t change resistance automatically. Here’s a screenshot of a Facebook marketplace search for “Tacx Smart Flow”, showing there are loads of them out there, now so many people have moved onto direct-drive trainers.
Speed sensors were a common recommendation a few years ago but now there are so many second-hand smart trainers available at such a low cost, it’s a no-brainer to go for a smart trainer as a newbie.