I’ve always been jealous of people who grow lots of plants. So I bought a plant from a garden sale. The lady who sold it to me said that it is a pencil cactus. Also there are a couple of plants that she said are called mother of thousands in the pot as well. (They’re rather small.) She said to water it once a week and that was the extent of her instructions. The internet says that twice a month should be sufficient. 
I’ve never really tried to grow anything before so mostly, I’m just looking for general advice and reassurance that I’ll be able to keep the plant alive in my dorm room. I’d hate to think I wasted those three bucks. If anyone has something to share please feel free to do so.
Anyone who tells you to water on a schedule, whether it be once a week, twice a month or anything like that, is doing you and your plant a disservice. There are too many variables in plant care, like light, temperature and humidity for the same set schedule to work for everyone.
Pencil cactus (probably not a cactus at all, most likely Euphorbia tirucalli) and Mother-of-Thousands (could be one of several Kalanchoe/Bryophyllum species) are succulents which do not need a lot of water. Let the soil dry out almost completely (stick your finger in the pot to your first knuckle to get a feel for things) then water thoroughly (until water comes out the drainage hole–discard the excess) then don’t water until the soil dries out again.
Give them as much light as you can. Direct sun is good, if you’ve got it.
Eventually, you may discover that you can predict how often your plant needs water. Then, maybe you can narrow it down to once a week, or twice a month or whatever. But it really takes knowing your plant and your conditions well.