Hi Adam,
If you are just wanting to rank your keyword terms according to the number of searches per month, you can use any of a wide selection of free or paid keyword research tools.
Probably the most well known and commonly used is the Google Adwords Keyword Tool. In order to get the estimated number of searches per month for the exact keyword terms you are seeking, type the list into the Word or phrase input field at the top (one term per line). Check the box underneath the input field that says O****nly show ideas closely related to my search terms. When you click "Search" you will see two separate lists. The first one will show you the exact terms you entered. You will now also see an option on the left of the screen to change from the default broad match to exact match or phrase match (I would normally be looking for exact match and local searches - USA by default). You can sort the list numerically by clicking on the column heading.
To download just your list of terms, check the box to select them, then click "download" and choose "selected" from the menu that opens. This will allow you to save the list in a number of formats, including csv for Excel.
The second list you see is a selection of "keyword ideas" which Google thinks may be closely related to the terms you entered. If you want to add some of these to the list you are downloading, just check the boxes before downloading "selected".
If you want to change location (you are not in the USA), or just look at search numbers for mobile traffic etc, click the Advanced Options and Filters link underneath the word and phase input field.
You should always keep in mind with any tool of this nature, that search numbers can never be exact, but it will certainly give you an indicative feel for the popularity of your terms. There will always be variations in search traffic from month to month, there are seasonal influences etc etc.
As Ryan indicated, there are a number of other factors to take into account when assessing the keywords you are going to target and the Whiteboard Friday video he suggested is a great place to start.
I would normally suggest that you also use the SEOmoz Keyword Difficulty Tool to help refine your list according to how competitive your terms are, but I believe there is an issue with that tool at the moment. The SEOmoz team are working on that, and you could make use of it once the issue is resolved.
Hope that helps,
Sha