One day chicks will be chickens. If you don't have a chatzer where they can scratch around, facilities for a large outdoor lul, or if you haven't arranged such a destination elsewhere in advance - please don't buy chicks!
As the chicks grow, they can receive table leftovers. NOT meat, but noodles and half eaten sandwiches are fine. If/when you provide whole grain you must also provide grit. This is essential for digestion if the birds don't run around on the earth. Grit and pellets for grown chicks are available at petshops.
(Pesach is a real problem for chickens. You can use commercial dove seed, checking it for the five grains- (esp oats). Most bird seed is kitniyos, and permissible for use with pets). Pellets are usually chometz. Make sure you know how to deal with pets on Shabbos! (muktzeh, tzad etc)
Birds generally do not like handling- it is extremely stressful to them. Handle only when necessary, very gently and briefly. Otherwise, leave them to themselves! In time a bird can become tame, but we can't impose ourselves on them.
You can keep chicks in a large cardboard box- on a bed of newspaper (change daily) or wood shavings. (ask for "nessoret" ) Never allow very young chicks to get cold.
They need careful and regular handfeeding.
Chicks of domestic fowl can feed themselves, but chicks of songbirds, (including sparrows) are fed by regurgitation for at least a month after hatching, though they will begin to feed themselves inadequately before that. In the unlikely event of a successful rearing it will be nearly impossible to rehabilitate this bird to the wild.
BTW Wild sparrows, pigeons esp. carry many parasites and diseases, some of which can affect human beings and their pets. Dust from feathers and droppings can aggravate asthma.