I use

It is run by google, so if you have a gmail account already, it will be simple to set up a blog there.

Edit:
I guess I misinterpreted your question, you want to know what to blog about?

The best advice I can give you is to blog about your expertise. Educational blogs can grow to be very popular, so, if you are a good cook, share some recipes; if you are a tech-savvy person, post about computers; if you are a creative person, post stories or poems.

The point being, the long term success of a blog is not really determined by the subject, but rather by how well the author knows the subject and how well he/she presents it to readers. Also, a little advertising among your friends or on yahoo! answers never hurt.