This is a very hard question for parents, because of course you and your child won't always feel the same about his/her choice of companions. It depends on how old your child is too. The older your children are, the more difficult it will be to influence his or her behaviour in this way, and indeed a friend who a parent sees as a 'bad influence', for example, may even be more attractive to your child on that account, especially as your child reaches adolescence.
The best thing you can do, really, is to try and encourage your child to appreciate the good qualities in people, to respect people who are kind, loyal, intelligent, hard-working etc from an early age. Then hopefully, even if he or she goes through a rebellious stage (which most young people do) it will not last too long and your child will essentially value people who make good friends. But ultimately your children will have to choose their own friends. Good luck.