How to secure a job in NYC?

You have your priorities backwards. First you must obtain a legal resident status or a H1B visa and then you can look for a job, not the other way around. (In case of H1B your employer "sponsors" you.)

It depends on what kind of job you are looking for. You should earn extremely well if you planning to live in the city. Finance Jobs would be the best.

1) have a masters degree or PhD or US citizenship. Unless you are an US citizen you cannot just live and work in the US. You will need to get a work visa and you will find it very hard to get one unless you have at least a masters degree. In order to get a work visa you need to have secured employment since your employer needs to sponsor you for a work visa. The most common visa is an H1B and unless you work for an university only a limited number of people can get one. If you work on an H1B visa, you are only legal to stay in the US as long as you are employed and while it is not impossible to switch employers, your visa would need to get transferred to another employer which is additional headache.

2) there are many people living in NYC, many of which want a second job since it is an expensive place to live in. So it is hard to get a job.

3) NYC is expensive, similar to London compared to the rest of the UK. Unless you have the necessary income, it's hard to enjoy if you are constantly broke.

To move from the UK to the US, you'll need a work visa. Have you factored this into your plans?

There are millions of jobs of all kinds in NYC. There are a million and one people looking for those jobs who are already US citizens. Your likelihood of finding an employer who will hire you and apply for a work visa for you are slim to nothing, unless you are extremely well credentialled in an esoteric field.

If you aren't, then I suggest you not waste your time looking for work.

You're more than welcome as a tourist however.