what programme to use?
Depends on where you are with current fitness and the length of climb you’re targeting. The most common method is follow a plan like Sweet Spot Base I and II, then probably general build (or sustained power, depending on your climbing goals.)
What fitness do you already have? What are your target climbs? Have you done structured training before? The more detail you can add, the better the advice can be!
This has a bunch of suggestions depending on your preferred discipline.