You can create a NEW POST depending on where you are on the blogger interface. You can post here (see arrow):
You will then have to put a POST TITLE, write on the body, PUT A LABEL and then PUBLISH.
A label is important to organize your posts. These will prove useful when you make your tabs on your main page. You can change the labels later as well as have multiple labels.
Here is a video to help you further (it does not have audio, just in case you are wondering):