There are several tools out there for keyword research. I see questions on here all the time asking how accurate those tools are. There is no way to be sure because none of them get their data from Google. These tools are great and you will actually need to use them to begin keyword research.
Once you have used these tools to come up with a list of keywords you will want to know how well they do in Google. If you already run adwords you have this data but not everybody has a big adwords budget.
Take your list from your research and set them up in adwords. Make sure you create good ad groups and landing pages like you would for any adwords campaign. The goal is to keep yourself below number 6 so you don't get many clicks but you get the impressions. You still have to follow normal adwords rules because they can lower your QS so that you don't show up at all. Set up all keywords as exact match or your number won't be accurate. What you are looking for are impressions. This will tell you the volume of each keyword.
You will need to monitor reports in adwords to see if you are showing up 100% of the time. This can be tricky because Google is always trying to get rid of people who don't get many clicks. You will get clicks but you won't be spending a ton of money like the people in the top 3.
A side benefit is that if you do this right you might end up ranking in top 3 with a very low bid.