Initially, your blog should be where you begin creating content, whether strictly written or a combination of written, audio and/or video. Once you have a range of posts that show your expertise in a targeted area, then you can approach others about guest blogging. Who you choose is up to you. Find people of similar interests with a sizable following to help get your blog visibility.
Another option available comes from LinkedIn. The social network recently opened up its publishing platform to anyone with a profile. Once published, it gets circulated within their Pulse network where you can then promote it among your connections.