There are tonnes of websites that have covered this question a million times over, I suggest doing your own research All in all Google keyword tool is a good starting point before moving onto paid tool like market samurai , keyword discovery, seockpit etc

just to name a few

The competition it is referring to is how many advertisers bid for that keyword. Obviously a keyword like "computer" isn't very specific and so has poor conversions. The user could be searching for computer pictures or other unrelated things that will not produce any sales.

One and two word keywords always have high search volume, doesn't mean it's a good keyword to target. Only testing will tell.

Global searches are the total number of searches around the world every month. The local searches are the searches in the country that you've chosen every month.