Welcome to the forum. Do you want to teach your boxer protection if so I would consult a trainer as DM_Boxers recommended? My boxers will let me know by barking if someone is outside the house. There are suggestions in the training section of the forum on general obedience.
what age is your boxer? Many boxers don't start barking until they are at least 1 year old. When they are babies they are more friendly with strangers, but as they get older they will naturally start protectecting you more.
I read that real quick and I thought you were asking how to get your dog to not be friendly to white people :lol:
I've been teaching my dog to bark at people outside. If you actually start making it a big deal when someone is outside and making a barking noise yourself (someone on the board suggested this and it sounds crazy but works), then praise and treat them when they copy you, they get it eventually. I think Little Boxer may be right though, the natural protection instinct may not kick in for quite some time. People looking for a guaranteed watch dog should probably adopt an adult dog that already exhibits those tendencies. But I'm sure with proper training, you can get them to do anything.
Unfortunatley boxers are VERY friendly by nature so trying to teach them not to be i imagine is going to be very difficult. Welcome to the forum. I would start with the obedience training and go from there.