C(1)-Eb(3b)-G(5) (C-Minor triad)
Remember that 1-3-5 is based on the number system as explained in another article
From this you can invert the root like this:
Look below to see in real time:
In a major key, your 6th tone will always be the relative minor of that key. So in the above example, C-minor is the 6th tone for Eb-major, therefore your relative major key is Eb.
So when a song starts with C-Minor you can actually start playing in Eb Major, but the only difference is that you start the song on the 6th tone and not with the normal 1 chord (Eb-major). Here is an example of how it would sound within a song context: