I'm going to introduce you to my two favorite ways to do keyword research, and neither require you to have existing site traffic/organic search data! If you're starting from scratch, pay attention.
The Free Route
Go to www.answerthepublic.com
Type in your keyword and you'll get a list of questions people are asking on the internet using your keyword. Why is this awesome? People asking a question are in need of answers, this is also a list of long-tail keywords you can use as exact match keywords for much higher relevancy. You're welcome!
The Test and Learn Route
Want to get started right this second and let user behavior lead the way? Build your AdWords campaign with broad modifier keywords like +wedding +cake for wedding cake vendors, or +boudoir +photos if you're a ... niche photographer. These will show your ads for all searches including these keywords in any order. Bid low, as this traffic will be unpredictable at first. But as you start to get these lower cost clicks coming in you'll be able to see the exact searches in your AdWords -> Keywords -> Search Terms tab and can use these as your exact match keywords! Boom! Relevancy! This also gets your ads out there right now, and lets you collect your own data. Honestly, either approach will get you started off in the right direction, it comes down to your comfort level. What sounds better to you?
Get in touch here and let's talk!