While you can train on a turbo with nothing more than your bike, it’s pretty mind numbing and as has been said just getting on and spinning away for twenty minutes will guarantee that you’ll do a session or two then give up.
Some form of variety is essential, how you achieve it varies on how much you wish to spend. At the simplest having some music playing out and doing an effort for one track, ease off for the next and so on will work (providing you aren’t doing the effort to Tubular Bells!), using the random facility on an MP3 player keeps things a bit more interesting and is similar to the randomly spaced efforts whilst out on the road/trail. You could do things like sprint during the guitar solo or chorus as variations.
Next up are the workouts that you can set up on GPS units like Garmins. Depending on what extra kit you have (HRM, cadence sensor, power meter) then you can create a workout based on levels using the values returned by those devices.
Trainerroad really does rely on power readings and while you can get away with virtual power, a dedicated power meter is best. TR then gives you a series of workouts as part of a longer term plan.
I find I need to build up the time spent on the turbo – you are sat in one position for the time on the bike and there’s no coasting.