Dragon fruit are a jungle type cactus so prefer more water and cooler temps than regular cactus, but let them dry out between waterings. They can fruit when the plant gets about 5 feet tall assuming it's been fertilized with a good bloom food. Since you will be keeping it in a pot, a sturdy tomato cage will work as a trellis, but you can find many different suggestions via a Google search. You can plant the broken off piece, but be sure you plant the bottom end and not the top, even though there are roots coming out of the top. Your variety may or may not be self fertile, so eventually you might consider getting a second variety so you can cross pollinate.

Kevin

I read that they'll bloom after they reach 10 lbs. Some are self-fertile, others need another variety.

We have several varieties, but only get a couple blooms at a time, always from (apparently) the non-self-fertile plants. We originally planted them outdoors, but they did badly in our rainy environment, so now they're taking over the greenhouse.

No need to keep the plants straight. The couple of fruits that are developing are on more or less horizontal plants.