Thanks for your question! Well, I have to admit that I’m kinda muddling through this myself. I like Tumblr because it’s very easy to use, especially in terms of letting people share your content. It’s basically the Apple of blogging. However, just like Apple, it comes with some constraints in terms of what you can do with it. If you want to get super fancy in terms of customizing your blog, I think the general consensus is that Wordpress is the way to. Also, as the name would suggest, I think Wordpress users tend to write more lengthy articles. Tumblr is more about the animated gifs, lol.
In short, my recommendation is that if you think you’ll write more than two paragraphs, and/or if you know a lot about HTML, I would go with Wordpress. If you think you’ll be more often writing shorter posts, and if you want people to be able to share what you write easily, I would go with Tumblr.
As for Facebook and Twitter, they are tools that can help you share what you post on your blog. You can even integrate them directly into Tumblr (and Wordpress too, I presume) so that the title of your post automatically becomes a tweet or a Facebook update.
I hope that helps! Blogging is a lot of fun and can even help you professionally.