Remember: all the time. Two-year-old, happily playing with the other children in the sandbox, will not consider someone as a potential friend or girlfriend he's too young for this. Accordingly, senseless will be the advice of parents: "would You be friends with Petya, he's so well behaved and calm!" or "be Friends with Dolly, such a good girl!" The kid just didn't know what he should do. But to tell him about the friendship it is necessary!
Read stories and nursery rhymes, watching cartoons with the child, which says about friendship. This is to ensure that the baby's head appeared a clear thought: Friendship is good! Let him remember the word and its mention will get positive emotions, because this is very important.
Three-year-old baby can more or less clearly determined: who he wants to be friends with and who - no. At this stage, show tact and patience, remembering that the logic of an adult and a child are completely different things. For example, situation: your child wants to be friends with good, kind boy, because of a defect. And a child answers the question "Why is he not friendly?" - "Because he's ugly!"
Gently but firmly reassure the child that moral values have nothing to do with appearance. There will some kind of cautionary tale (like the "Ugly duckling"), the parable or story of life.
When you become a little older, be sure to explain to him, what is the value of true friendship. The task of moms and dads – to convince his offspring that other have to help, to share with him, to keep from bad behavior. Not asking anything in return, without asking the question: "what do I have?". Then the other will come with him in the same way.