I am a naturalized citizen. I am planning to adopt my nephew who is in Philippines. Would I be able to bring him here in america? Or is it easier if I petition him as an alien relative first?

You can file for immigrant visa for him if he has been adopted for at least 2 years prior to filing, plus you need proofs that you have been supporting him in all aspects during the past 2 years.

Adoption in US is limited to the age of 16, so if he is over 12 now, forget it. The Philippines has the longest backlog, you might have to wait for 5-6 years and he will be age-out by then. Most important thing, approval rate of immigration cases on adopted children who are not orphans but related to petitioners are near zero.

If you want to bring him to school in US, better get student visa and you can sponsor him. The only issue is only private schools can issue I-20 for foreign students. They can't go to public schools.. except those who come on exchange programs which are limited to one year only.