I must admit, this depends on your circumstances, especially given the fact that ralts is a branched evolutionary family. While there are certainly similarities between how you would raise a ralts who will eventually be a gardevoir and a ralts who will eventually be a gallade, when you get to preparing for their evolutions, the training regimens you will need will undoubtedly be different. Likewise, as soon as your ralts evolves into a kirlia, you may need to adjust its lifestyle, depending on whether or not it wishes to evolve at all and what it wishes to evolve into. A gallade requires a diet heavy in protein (to build muscle) as well as exercises of a more physical nature (especially to build arm strength). Meanwhile, kirlia that don’t wish to evolve and kirlia that wish to evolve into gardevoir will need exercises geared more towards developing their psychokinetic abilities, including more meditation than actual physical fitness. Additionally, as they develop their psychic abilities, they’ll need to rely more on a plant-based diet to avoid consuming anything that could experience emotions upon death, as the psychic exercises may increase their empathic strength.
Speaking of which, that’s the main challenge in raising a ralts at all: the empathic factor. Young ralts especially will be a little uneasy as they get used to feeling your emotions, and thus, from an early age, they should be trained to build psychic barriers. Keeping other, older members of the ralts family (including gallade) could help, but really, any psychic pokémon capable of using telepathy will do. Younger ralts will also require a lot of meditation in order to teach them mindfulness, which in turn will help them control their own emotions and prevent themselves from projecting onto others (including you).
A lot of these exercises may consume much of your ralts’s time, so distractions for stress relief are absolutely necessary. Soft toys may be a good choice due to the comfort the ralts may derive from the way they feel, but be wary when getting anything with a face. Ralts who struggle with controlling their psychic abilities may inadvertently bond with their stuffed toys, which increases the risk of producing a shuppet if the toy ever needs to be discarded. This doesn’t mean you shouldn’t give a ralts a stuffed animal—just that if you do get them one, don’t let them or the rest of their team beat it up too much, and be sure to have them draw out any psychic energy when the toy is ready to be discarded. Otherwise, believe it or not, but stimming toys normally given to humans are also excellent choices. Moon sand as well, for that matter. The more a ralts’s physical senses can be engaged when playing with the toy, the better, as it allows it to focus on the physical realm, rather than the psychic. (They’re also rather fun to collect or create, but … that may simply be my own personal preference showing through.)
In terms of food, it’s a misconception that members of the ralts family are exclusively vegan. While they can’t eat meat that had been taken from a living animal, eggs and dairy are perfectly fine. Dairy, after all, is not a complex living thing in itself, and eggs have never reached the stage where they would have developed emotions. If you’re thinking of evolving your ralts into a gallade, as I’ve mentioned earlier, protein is a necessity, so eggs—along with vegetarian proteins such as nuts or soybeans—are an essential. Otherwise, ralts that you don’t wish to evolve or ralts that will eventually become gardevoir can be completely vegan—if they wish, anyway. It’s always a good idea to ask your ralts what it prefers.
Otherwise, taking care of a ralts is very much like taking care of a small child. They sleep in small beds (with blankets and a pillow, preferably), they can be toilet-trained, they bathe in similar ways to humans, and they often need the same sorts of affection we give children. It’s just that it’s very highly recommended that you avoid physical contact as much as possible with particularly young ralts until they develop the aforementioned psychic barriers, as projection from a psychic pokémon is not a particularly wondrous experience. Thus, as tempting as it may be, your ralts will really need its own bed, and you may wish to avoid hugging it as much as possible at first.
Best of luck, anonymous.
The Pokédex According to Bill 