I'm going to start with some tips for beginners. I think a great way to start blogging, is to write some things about you. It can be as simple as your likes and dislikes, hobbies, or even the pets you own. Just remember, never reveal your personal details, like your address, phone number, or full name. I also wrote a blog post about not putting your picture on your blog, which you can access here. Another tip is to remember that a blog is not a journal. Try not to overuse phrases like "and then I", or "so then."
To me a good blog subject is anything that will keep your reader interested. Try not to write too much about one thing, because it is likely that your reader will get bored. If a lot of things happen to me in one day, I try not to write a million blog posts about those things, but instead write one blog post about my day, explaining all of those things in it.
There are many ways to successfully blog, but to me the main one is to get someone to check your post for spelling errors. Don't you just hate it when you get to your favorite blog, but there are so many errors in the post you are reading? If the person is someone you know, or is in your class, offer to edit their post.