ages 2 to 4 is the most impressionable age of a child, most things he sees and is taught at that age will later surface at age 10 and over....

How old? You have to get them to put themselves in another persons shoes or remind them of a time when they were on the other end of someone elses selfish behaviour. "Remember when James ate all the cupcakes and didn't leave you any? You felt pretty angry didn't you? So why did you just eat all the biscuits and make Ruby miss out? How do you think Ruby feels now?" Also books and sad kid movies are great for learning empathy. Model helping others, when you see someone that needs help offer it and then discuss with the child afterwards why you offered help and how the other person would have felt when they were offered help.

They learn at home your values. Change yourself first.

If they are quite young still, show them empathy and selflessness, and narrate your way through it.
For example, oh there is only one pieces of chocolate left, I really want it, but I'll give it to you as I can see you really want it to.
Are you struggling to put your shoes on? I'm really busy, but I can help if you like as we should always help those around us if we can.