For most people starting record labels, the idea to get a label going in the first place came from hearing some great music that no one else was putting out. If that's the case for you, fantastic - move on to the next step. If you just have the idea for the label and need some music to get going, now is the time. You will need to have a release - or even a few releases - lined up to move on to the next steps, like finding distribution and PR.
Finding music to release can be harder than it sounds - it's a bit like finding the proverbial needle in the haystack. One of the easiest thing you can do is to start locally. Go check out some local musicians and see if you can find a few artists you'd like to work with. You can also listen to some music on MySpace, Bandcamp, ReverbNation and other sites that featured unsigned acts.
An indie label is a labor of love, so it is really important to hold out for some music you *really* believe in. When you've decided to start a label, you can feel like it's pretty urgent to go NOW. In the long run, waiting until you have a record you love and can't wait to bring to the world is worth it.