Here are some ways to encourage your child to make friends:
Try to provide regular opportunities for your child to play with others.
For younger children, consider joining a play-group or meeting with other parents who have young children.
For school-age children, try to encourage playtime outside of school - perhaps suggest your child invite friends home, or sign her up for after school activities or teams.
For children of all ages, try to include both group play and one-on-one time.
Allow your child to pick his own friends. Try not to force friendships on your child.
Remember that a child's temperament will affect the way she plays with other children. Some children might be slow warming up to others, whereas other children are outgoing and fast to approach peers. If she is slower to warm up, then provide some extra time for her to get acquainted with others and encourage smaller group play - your child may feel more comfortable with less children.