I suggest you read some of the older posts in this section of the Expat.com Forum. Many people had similar questions and a lot of similar topics are there.
Unless you have some special qualifications/education that allow you to get a job that pays a decent salary (and those jobs, even if you're qualified are hard to find) you won't be able to make enough to live from. Simple jobs pay the minimum wage of $ 7.25/hour and you will only get a part time so you'll need multiple jobs.
Next to that, although in San Juan you might get lucky, you should be fluent in Spanish.
Bottom line: if you don't have a nice financial buffer to set yourself up with the basic needs and keep yourself alive for the first year or so, it's going to be almost impossible.
Now, mark my words, there are going to be people who will tell you that the lifestyle here is wonderful and that you should follow your heart and go for it (without telling you how to survive, of course).
My advice is simple, if you don't have a financial buffer (and I'm not talking about 5 Grand but more 20, 25 at least) don't even start thinking about it.
If you want to start a business you will need more or a lot more depending on what kind of business.
Some call the above negative - I know it is realistic.
Don't get me wrong, Puerto Rico is a nice place to live if you have sufficient income. I've been here for quite a while, have a business that provides for me and my family and I'm really happy here.
The problem is that it's really hard to get a job, also for locals. Lots and lots of young people are going to the mainland USA because they can't make a living here! Even if you're lucky to get a job and even more lucky to get full time employment you will still have a real hard time making ends meet.
There are of course ways to get started here but that requires some planning and preparation.
One way would be getting a federal job in the USA with an agency that is also present in Puerto Rico and try to get a transfer. The same goes for getting an education that can get you hired by a company that has offices in multiple countries.
Another way could be to start an online business for which you only need internet access and - if you're selling something that needs to be shipped - access to the USPS and/or Fedex/UPS etc. I know more than a couple of people who make a decent living with an online business, both here on the island and in other countries. Of course it takes time, a lot of hard work and money to get any business running.
One way or another, make a planning of how you're going to make a living and make sure you have a financial buffer.