+1 on that. Spin bike is best if you won't ride outside at all.I am with GreyGhost on this, most cyclists will use a real bike mounted to a turbo trainer. That is what I do. There are a lot of smart turbo trainers these days that have the online training sessions etc. Better option IMO than a spinning set-up, but to each their own.
Otherwise you use your pre-existing bike on a trainer. Could even use a fluid or magnetic resistance trainer that's super cheap. Or you can go super fancy and get a direct drive trainer...a lot of them are still cheaper than a good spin bike and provide a much better "road feel"