There are a few things you need to get,
1) backflow prevention device off of the main.
2) proper sizing.
You are better off with larger pipes.
This link should help
Rainbird for do-it yourself home page
You should check with the local inspectors to find out what backflow prevention devices are accepted.
If you are going from copper to pvc, you would use a copper female adapter and thread a pvc male adapter into it. This is critical. It doesn't work the other way around.
Male pvc fittings can go into metal female fittings, but metal male fittings should not go into plastic female fittings.