The first thing you can do is understand and sympathize that changing high schools is a major and stressful change for your teen. A teen's social network is his/her entire world and going to a new school means learning an entirely new social structure -- on his/her own. So be patient, be supportive, and expect to see some mood changes in your teen, such as increased moodiness, irritability or anxiety.
In order to provide your teen with some practical help, try to get as much information as you can about the new school before your teen actually starts attending. Ideally, your teen (and probably you as well) should have a tour of the school, school handbook, and maybe a sample of the school's newspaper, prior to your teen starting classes. The more your teen knows about the new school, the less he/she has to figure out "on-the-job." Many schools have Internet presence as well, so take advantage of those resources. Since you're new in the neighborhood as well, have some social gatherings with neighborhood families, especially those with teens, which can help your teen meet some peers outside of the school setting. If you arrive during the summer, get your teen involved with local activities where he/she is likely to meet other teens as well. Once school does begin, encourage your teen to get involved in the same activities he/she enjoyed at the other school. Remind him/her that while the school has changed, he/she hasn't, so he/she should do the school extracurricular activities that he/she has always liked. You should also remind your teen that he/she has navigated through a new school before (at the very least, moving from middle to high school), and he/she can be successful at it again.
Finally, try to keep as much of the same household routine during this transition as you can. With all the changes and adjustments your teen will need to make outside the home, having as much stability and familiarity with his/her home life will be a comfort.