Do I have to declare anything when I come?
I will be attending Medical school and applied for resident status on the Island. I will be bringing my personal laptop and some items like: I own 4 watches and used books.

Bill, there is no customs form to fill out.
Once many years ago, before I moved here, somebody at customs asked me what was in my bags and I told her, but in recent years I believe customs is focused on intercepting drug smuggling. They sometimes x-ray bags upon entry.
As long as you bring your items for your own use while you are in medical school, I do not think you will have any duty to pay.
If you are granted resident status, you will be allowed to bring your possessions (a whole cargo container, including a car if you wish) without paying duty provided they are not newly purchased. Check the Sunbelt Realty web site for details on those rules.
Enjoy your studies here!

Thanks!

4 watches! I guess you'll be getting to class on time!

Yea 2 for each wrist LOL ;)