How to test how good a Niche is?

I have "acquired" a list of several hundred niches which are all suitable for my website, I was wondering how do I find the perfect keywords for the niches and how do I find which of the niches have the least competition but will still give decent traffic? noob friendly methods please.

nobody? :3

Keyword Research tools.

I use Market Samurai, but I've heard Long Tail Pro is good too.

It's called keyword research and you can start with google keyword tool and Traffic Estimator.
You can also buy a keyword tool or sign up to a service.
Add your list to the tool(s) and you should get a answer to your question...

In general, I am looking for a keyword phrase that has between 1000 and 2000 Global Monthly Searches. Also you should focus on buying keywords: buy, where to buy, cheap, reviews, best, affordable, bargain, comparison etc. I think it's best to target USA traffic.

Type in Google: allintitle:"your keyword"
0 - 200 : Extremely Low Competition
201 - 1,000 : Low Competition
1,001 - 2,500 : Low - Moderate
2,501 - 5,000 : Moderate
5,001 - 10,000 : Moderate - High
Try to focus on niche that has competition under 2 000.

Now type in Google your phrase with quotes, example "your keyword"
<20,000 Pages : Very Easy Competition.
20,000-100,000 Pages : Easy Competition.
100,000-350,000 Pages : Medium Competition.
350,000-800,000 Pages : Medium/Hard Competition.
Try to focus on niche that has competition under 100 000.

Look at sites on the first page in Google. You should evaluate PageRank, number of links, etc. You can use free tools for your research: majesticseo,opensiteexplorer,ahref,google keyword tool etc.Or you can buy MarketSamurai,LongTail Pro etc.

EDIT: You can read this BHW thread The Absolute Newbie's Guide To Keyword Research.

how do I pick a single keyword? for example if I choose a game and I am going to try and promote a coin generator so the keyword/phrase would be "game coin generator" how do you choose a single keyword for this?

OP

Did you read AND absorb the post from plb009 and others above?

If you really want to understand, then try to think of how to smoothly do the research,
in a time frame which will give you results more effectively.....
there are tools to help with that, just read the threads, and try also the BST sections,
think about how they do the research, think about how YOU could do it for yourself,
have you tried checking out the many many YT videos? etc....

What I am saying is simply this...
The answer to your main question (and subsequent follow ups) lies in this very forum,
do some RESEARCH yourself, and kindly read the many thread and posts,
members here have already posted the ongoing answers and methods SEVERAL TIMES,
no one is going to articulate it for you in any more simplier terms,
everyone has to do it for themselves (if you really want to know and succeed that is)

JUST DO IT!

Good Luck