This will create a new post for you and take you to the post editor, which looks like this:
Add a title for your post in the box at the top. Then write your post in the big white space. Blogger is fab because when you're writing your posts it's just like writing them in word. Most of the little buttons along the top are very similar to word. Below I explain what each of them does.
I recommend that you keep in compose mode as you write your post. You can click on html, and it will show you the code for your post. This is what you would have to write if you didn't have all those helpful little buttons at the top to format what you type. It can be useful sometimes to have a look at the html, but not generally for writing a straightforward post.
Once you have written your post, click the preview button at the top to take a look at the post in situ on your blog. I often do this and have a read through, and sometimes I think that it's not quite spaced out properly or that I'd like to rewrite a bit. To make changes you just go back to the editor window (the profile helpfully opens in a new window).
When your blog post is finished, click publish. This puts it on your blog for everyone to see. But don't worry if you want to change something later, you can simply go to your list of posts, hover over the title and click edit.
Now to each of those little buttons. Here is what they do. Remember to select the text you want to change first, and then click the button.
Add a link to selected text (more info here)
Add an image (more info here)
Clears all formatting on selected text - very handy if you've been playing around and it's not quite gone right!