Blog stands for Web Log.. also known as an online Journal. Basically all you do is write in your blog like you would a journal. You can write about anything at all. It is really easy to do. First you will need to sign in to the community.
Then: Click on MY STUFF near the top of the site on the purple navigation bar. (This is where you can add to your gallery or blog) You will want to click on NEW POST TO __your name___. This is where you will post your blog entry. You will see an area for TITLE and BODY. Write your blog here and then click PUBLISH at the bottom right. Congrats! You have posted your first blog!
Don't worry about tags right now. That is a subject better left for another time.