Watering
Watering is the part of indoor plants that gives the most trouble to newcomers, and also the part that improves the fastest with deliberate attention. A few weeks spent on watering carefully — rather than rushing to the next thing — usually outperforms months of unfocused practice. The improvement is not glamorous and rarely shows up in a finished result anyone else would notice, but it is what separates a frustrating hobby from a satisfying one.
The rule of thumb: if something feels off and you cannot say why, the answer is almost certainly in watering. Slow down, observe, and only change one variable at a time. Keep brief notes if you can. After a few sessions you will start spotting patterns that were invisible at the start, and watering will stop being a problem.
That covers the basics. Beyond this, indoor plants opens up in different directions for different people — some go deep on pet-safe choices, some on light, some discover an area not covered here at all. All of those are fine. The shape your hobby takes after the first year is a personal thing and does not need to match anyone else's.