Updated: Sep 23, 2021
I was really bad at keeping plants alive. I think just the act of me buying them killed them. If you are wanting a plant to keep indoors for the first time I suggest a cactus or succulent, a sunny window, and the occasional watering.
Cacti and succulents are very forgiving. Cacti especially. They don't need much water and are slow-growing, so if you are pretty forgetful they will survive or just grow slower. and if you mess up you can usually salvage them by cutting the good parts off and try to propagate them.
I have had my thornless prickly pear since the beginning of 2014. Maybe it is not as big as it is supposed to be, maybe went a month or so without water a few times, and maybe it was left out in the PNW winter a few times. But despite that, it is alive and thriving. Well...the piece that survived the winter is at least.
You can get a cactus pretty much at any grocery store garden area. I like to go to nurseries just because there is usually more of a variety. They also label their plants better than "cactus variety" because I am not the greatest at identifying plants.
Once you have your chosen one, you can re-pot it in a container that is slightly bigger than the one it came in. I like to use pottery or terracotta with a drain hole because it absorbs and drains excess water. Yes, you can over water cactus and succulents easily. Less is more with them. You being forgetful is not such a bad thing with this plant.
Don't forget to re-pot it with cactus and succulent soil, or a mixture that drains well. Not peat moss, not straight-up soil or compost. If you only have potting soil, it is OK. You can mix it with sand and perlite. I would do 3 parts potting soil, 1 part perlite (or small bark chips if you don't have perlite), and 2 parts sand. This is because potting soil alone would hold too much water for too long and will cause a smushy unhappy cactus.
Now for the sunlight requirements... cacti, and succulents need full sun. Which means 6 hours of sun. South-facing is best with nothing that will block light or shade it. You can try other windows but there might be some leaning going on. In which case you will have to remember to rotate your plant to keep it fairly straight, about once a week. This is because the plant is trying to compensate and get as much light as possible. It will be worse if you have a filter or film on the window.
If you don't have enough sunlight you can get a grow light. You will have to do a little bit of experimenting to get it right but you want it fairly close to your plant but not too close that it will burn it. 6 inches is pretty safe. You might have to have it on for 8 to 10 hours since grow lights are not exactly like natural sunlight. A little goldilockish but whatever. Grow lights work great with a timer switch or smart switch. There may be a better way but this worked out for me.
I bring my cactus in for the winter and have it under a grow light and it does just fine. Just watch your cactus closely for a month. If you notice it getting burnt or leaning away from light: move the light farther. If you notice the plant itself stretching out, growing thin and/or long and/or leaning a lot towards the light move the light closer.
Once you have re-potted and have your cactus or succulent set up, water it. Don't drown it. If there is excess water in the dish, remove it. If you got a pot that doesn't have drainage holes just water it till the soil at the top is pretty moist and still draining, not soggy. When the soil is dry you can water it again which is usually 2 weeks to a month. You can water more in the summertime when it is warm. In the winter you will need to water it less since it will not be growing as much. Even though they are indoors there are still fluctuations in temperature and the plant knows. Unless you keep your house at a consistent 80F all day and night during the winter then maybe you will have to water more. And can you invite me over as well? XD
Cactus and succulents don't need a lot of nutrients so you can use a succulent liquid food or powder about once a year during the warmer/growing season and that should be enough. If you do notice it looks pale or growing slow even in the warmer months you can try doing it more often like once a watering until you see results, otherwise you might have a different problem like root rot or disease. That is rare. You can always go to your local nursery or online, don't forget pictures, and ask for help.
Here are pictures of my cactus when I got it (Don't ask where the companion ended up...) and 7 years later. Then a few pictures of it earlier this year indoors during spring.