Q: Which mission agencies can someone 18-21 years of age with a high school degree join and begin active service as a missionary?

A: There are many. Here’s a few.

Look at interdenominational agencies including:
Operation Mobilization http://www.om.org
Youth With A Mission http://www.ywam.org
International Teams http://www.iteams.org

A number of denominations have opportunities including:
Four Square http://www.foursquare.org > World Base