I try to remember to type anything that might be long or complicated into a simple word processor and then paste it into the Body window when I want to post, just so I don't have to start over if I go to some other page and lose what I was typing by accident. By simple, I mean something like WordPad or Text-Edit; if the word processor's set to use curvy quotation marks and asterisks instead of simple straight ones, for example, they'll show up as weird symbols on some people's machines.
Every Body window, where you can type what you actually want to say, has a link underneath labeled "enable rich text". If you click on that, you get a little menu with buttons above the window; they let you format what you're writing by adding italics, creating links, inserting pictures, and so on. If you pause the cursor above each button, a little label that tells you what it does shows up. Usually, you use the button by selecting some piece of text and then clicking the button to format it in one way or another. If this sounds good to you, I'd suggest enabling rich text, typing in a few lines of "The quick fox jumped over the lazy dog," and just fooling around trying out buttons and getting a sense of what they do. It will be more fun than trying to learn it while you're actually trying to say things too.
Enabling rich text lets you do a number of things just by clicking buttons, which is nice, but it doesn't always do what you want it to, which is not nice. (As far as I know, for example, you can only get single spacing in your rich text posts by holding down another key - Shift on my Mac - while you hit the return key.) And the rich text editor adds a lot of hypertext markup language (HTML) tags to your text which look like gobbledygook to you if you're just getting started blogging and you try to edit the text afterwards.
Tastes vary, but you might find it easier to just type and add your own tags. You put <p> at the beginning of each paragraph, and </p> at the end of each paragraph. You put <br /> wherever you'd like a carriage return. You put <em> at the beginning of what you'd like italicized and </em> at the end of it. That may well be enough to let you say what you'd like to say for quite a while, and it will mean that when you try editing something, it will look like what you were just typing, instead of having a lot of other mysterious tags in it.