keywords

I have a pretty general review site that I am going to be writing articles for and trying to rank and then monetize probably with adsense. I have some questions about my keywords though, the thing that has always confused me.

Should I have multiple keywords, focusing on a primary keyword and several secondary keywords?

If you have multiple keywords do you try to rank each page(blog post in my case) towards a specific keyword different from the others? For example in my case with my review site could I have somesite.ex/money and rank that primarily for money and then have another post somesite.ex/washrooms and try to rank that for washrooms?

Another question about keywords is to due with keyword research. I am using google adwords to analyze them as I have no money to spend on any special software atm. What is a realistic amount of Global monthly searches to be trying to rank for or is it more the "competition" that matters? For example I saw one keyword that had almost 2 million global monthly searches but the competition said low on it and that didn't make much sense to me. Should I be looking at other factors for my keyword research?

Thanks, post might be all over the place but I hope someone can help

You can go for one primary and a couple of secondary keywords to start with.