The beldum line tends to be a bit … mechanical in nature. That is to say, unlike many pseudolegendaries (dragonite, salamence, and the like), metagross is a relatively tame final evolution, and its preevolutions aren’t that wild either. At most, you may need to watch out for beldum, as it may occasionally use Take Down for no discernible reason whatsoever, but if you’re a new trainer, you’ll likely spend most of your time outdoors (and thus away from any potential hazards for property damage). On the other hand, you will also need plenty of healing items on hand at all times, as beldum only know Take Down until after evolution, which means the only move they can use is a move that also does damage to the subject. Moreover, beldum—as with many other pseudolegendaries—are notoriously slow to raise, so you may find yourself using Take Down a lot.
In terms of care, beldum and its evolutions need little. Most of their nutrition comes from consuming minerals around them, although you could feed them kibble if you wish. Otherwise, allow yours to roam around outside and plant itself occasionally for feeding. It requires no water, and as it’s a mostly mechanical being, it needs little in the way of distractions. It would be a good idea to read to it now and then, however, as this develops its mental abilities. Likewise, be sure to polish it frequently (ideally every day but more practically, every week), and apply a metal coat to it if you notice any cracks, dents, or other damage in its outer shell.
Other than that, there isn’t much else one would need to know to raise a beldum. They’re essentially one of the easiest pokémon to care for, even though they’re among the hardest for a new trainer to train. But so long as you have patience and plenty of money for a healthy stock of potions, you should be fine.
Best of luck and congratulations, anonymous!