Everyone's family situation is different, but honesty goes a long way when it comes to handling family matters like this. If you arrange a kind of formal meeting with your foster parents and tell them your wishes in a straightforward and honest way, they are likely to understand. Foster parents generally enter the foster parenting role with a high level of understanding to begin with, so they may already have a better level of understanding than you may be aware of.

I do stress though that you tell them you have something important that you'd like to discuss with them, and ask them to sit down with you away from everyday distractions to sit and discuss it. Reassure them that you're grateful to have them as your foster parents, but be honest about your reasons for wanting to meet your biological family. There are also some good medical reasons to meet your biological parents, as you may discover there are some medical issues that run in your family, such as a predisposition to heart disease, certain cancers, and alzheimers disease and the like. You should find out these things now, so it can be part of your future medical record.

Be sure to treat this matter seriously and professionally, and be prepared to accept their decision in case your foster parents disagree. Try to understand whatever concerns they have, and in that case you may need to wait until you turn 18 to carry on with your search for your biological family. If you don't have any information on your biological parents, seek out organizations that assist with helping people find their biological family, as you're hardly alone in this type of search, and there are good resources available to help you.

Just sit down and tell them. I guarantee you they have been expecting this and have been preparing for it. They would prefer, I am sure, to be involved rather than to be left out of your search. Be prepared for the possibility that they can't help you, however. Sometimes information about birth parents is very difficult to get ahold of.