Go on your profile, click "View Blog Entries" on the left (just under your username with a bunch of other links, it should be the last one), then click "+ Create New Post" in the top right~
Click on Blog Tree at the top of the screen (you will need to scroll up from this post), then on the right hand side of the page it takes you to, click +Create New Post. Once you do that a text box will appear. When you are finished writing whatever you want to blog about, click the Post Now button found directly under the text box.