The first thing I would do is line them up across from one another and have them throw some underhanded pitches back and forth. You need to identify a couple of pitchers as early as possible and try to get them started on some type of pitching routine. You could have the greatest HS team in the country, but without a pitcher you will never be competitive.
This won't be the *first* thing I do since there's a good chance some of them have never even tried to catch a ball before, but it will be one of the first for sure.
Should I give any kind of instruction or just say "throw underhand" and then grab the ones who look to have natural ability?