Nowadays I've heard of teachers and administrators being proactive in making sure a child who doesn't know a soul at his or her school feels comfortable and meets new friends.
Here is what you can do as a parent.
1. Talk about it. Address it directly. Ask your child: Are you scared? Are you nervous? And then, is there anything I can do?
2. Talk to your child's teacher. Make sure that teacher knows a new child is coming into the classroom. And hopefully that teacher will break the ice.
3. Join your child at the bus stop on the first day. Even if he or she is older (as I was going into my sophomore year of high school being the new kid) it still feels good to have mom and/or dad nearby as you head off for that first day. I remember attempting to "man up," but being pretty scared!
4. Don't express concerns if your child doesn't hit it off with anyone on that first day. If you start doubting your child, he or she will have even greater doubts.
5. Talk about it when he or she gets home.
I also found some advice for children to use themselves when they experience that dreaded first day in a new school. This link offers some pretty good tips: http://www.ehow.com/how_2383968_handle-being-new-kid-school.html
Most importantly, try to have fun. Once your child is settled in, this day will seem like it was a real cinch!