Ask-a-Dave's-Gardener: Need help with my new mini-cactus, 1 by jbelang

jbelang wrote:
At an event a few days ago, I received a small cactus as a souvenir/favor of sorts. I've always wanted a cactus, even if it is just a cheap one from the hardware store or something, so I'm excited to have it.

To begin with, I have no idea how I should be taking care of it. I'm keeping it on the windowsill in my apartment, where it gets sunlight during the day. I watered it once this week. I could really use some guidance as to how to care for this plant.

Also, I do not know how it was treated before I obtained it, but when I received it, it was tilting slightly to one side. As time has gone on, it has tiled more to that side. I've read about rot in cacti, but I don't know how I would tell if it were afflicted. Could it be the soil? What should I do about this issue.

I've attached a few different views of the cactus for reference.