How can I sign my Windows Store app from Window Store certificate?

I believe you only need to sign your app if you are sideloading it. If you upload it to the Windows Store, Microsoft will sign the app for you.

From the MSDN article Submitting Your Windows 8 Apps:

Signing and publishing. In this final step, we'll sign the packages you submitted with a trusted certificate that matches the technical details of your developer account. This provides customers with the assurance that the app is certified by the Windows Store and hasn't been tampered with.