At first, no one is going to want to cover you (without some kind of traction) or allow you to guest blog on their site. You need to refine your writing style and gain some recognition as a blogger.

Set up a blog (personal or directly related to your startup) and start blogging. If you need help with how to setup a blog, there are other guides and resources out there that can help with that.

Right now, you just need to start writing different types of posts and hitting publish. The first few will be bad, but you'll get much better with practise.