Is it possible to adopt my 14 and a half years old nephew from Thailand? I am CA Registered nurse and have green card almost 4 years.And my nephew will come to USA for studying in high school in this year. Thank you;Penny.

Generally there are a lot of requirements and regulations governing international adoption. For example, you must be a United States citizen.
In my view the easiest / and better/ way is to apply for guardianship.