Starting a blog? How do you know which format is right for you? Good question. There are tons of different places to set up and blog, but I'm only familiar with four of them so that's what I'll talk about.
I use blogspot, wordpress, live journal, and tumblr. All for different reasons.
Blogspot: This was the first blogging journal I set up. I choose it because it was easy to navigate. I use this for my main writing persona for both readers and other writers, and yes, I have also sometimes put personal posts on it.
I've noticed that the majority of writers I know also use blogspot.
Wordpress: I use this blog for my pen name CC James. I'm fairly new to it, but have found that it is as easy to navigate and set up as blogspot was. It's another favorite for writers. The appearance--fonts and layouts--is a little more cooler than blogspot.
Tumblr.: I also use this blog for my pen name CC James. I opened a tumblr., mainly because that's where all the hip cool young people hang out and since the CC James books are targeted for the younger crowd it was the place to be. It took a bit of a learning curve for me to figure out, but once I did it's easy. Also I'm no techno geek, so if I can do it, anyone can. What I love about tumblr. is the interaction ability. Once I subscribe to another person's post, their posts show up on my dashboard so I don't have to go hunt them down and vice-versa if someone subscribes to my tumblr, they don't have to hunt me down either. It is so much more interactive than wordpress or blogspot, which is why I think the younger crowd loves it. Kind of like an expanded twitter that you can load pictures or gifs and
retweet reblog other's cool stuff at the click of a button. You do have to put in the time to gain a following by interacting with others, which is part of the fun. But you have to do that with any blog anyway.
Live Journal: This is by far my favorite. I use it for an entirely different purpose besides my writing. The interaction is the same as with tumblr. I love that I can go to my "friend's page" and all my friend's posts are right there (under cuts) so I only see the first line unless I choose to see more. I don't have to go hunting all over the web to find them. It takes minutes to scroll through and see what everyone's up to and make a few comments. Also, there are livejournal communities that you can subscribe to and anyone who posts to the community shows up on my friends page as well, so I also don't have to search for people with the same interests as me. They are all right there. I can then decide whether to comment or friend them. Also on the flip-side, everything I post goes automatically to those who have friended me so I'm getting much more exposure than with blogspot or wordpress.
All of these sites are free. Livejournal does show ads, which is annoying, but you can opt to become a paid user for $15 a year and the ads won't show on yours or your friend's pages when you go to theirs. It's well worth it.