There are two ways for you to post to your blog.
#1: At the top of every site page is a "+Create" button. Click on that and select "blog entry". A floating box will pop up asking you to select the blog you want to create an entry for. Select your blog and click "continue".
#2: Click on "Forums" in the main menu bar, then click on blogs. Under the "Blogs" heading, on the right, there is a green "Add new entry" button. Click that and select your blog.
Regardless of which option you chose, you will be taken to a new blog post page. Give your blog entry a title, tags if you'd like them, select whether or not to "auto follow", and whether or not you want to share to twitter (providing you have linked your twitter to GA). Enter your blog content in the editor.
If you want to post immediately, then after entering your blog content, simply click on "Submit entry".
If you want to save as a draft, then click on the "Publish" slider so that it turns grey.
If you want to post date a blog, then under the Publish heading and just above the "submit entry" button, uncheck the box for "immediately". Fill out the date and time fields, and then click "submit entry".