What's the recommended kitten food? How often should I feed it? (It's a kitten abandoned by its mother after 2 months.) I give it milk around every 4 hours and twice a day give it minced meatballs. Is this sufficient (considering its age of about 2 and a half months old), or should I add something more to it?
I would avoid giving cows milk as there is a good chance the kitten will be lactose intolerant. Stick to kitten/cat milk such as Whiskas Kitten Milk or goats milk in a pinch. The meat balls might not be the best - if they are intended for human consumption then it may contain higher levels of salt than is healthy for a kitten and may be lacking in nutrients that they need. The odd "treat" of human food isn't really a problem but it shouldn't form the main bulk of the diet.
At two months old a kitten should be eating kitten-tailored cat food as it is generally more calorific then adult food to support growth. something like Royal Canin Kitten or Whiskas Kitten wet (the Royal Canin is higher quality but does cost more). You could also try giving some dry food such as Well Beloved Kitten dry.
These are examples and obviously there is a wide range of brands and flavors that exist - it should be labeled for kittens (they may describe it as being for cats under 12 months old) and ideally you want it to be described as "complete" - this means that it will provide all the nutrients that the kitten needs.
At 2 months old your kitten can already digest solid foods.
After weaning (that is, when they no longer get milk from their mother's teat), most mammals, including kittens lose the ability to properly digest milk (specifically lactose).
If you really want to give it something that looks and (sort-of) tastes like milk, go for the special kitty milk, which is not made of kitties, but rather from milk with reduced lactose content.