Domain name! First and foremost. If you have not purchased your domain from blogger yet then you should. Then you need to wait about 3 months before you can transfer the domain to your new hosting service. There are a couple reasons for this.
You need to get your blog off of blogspot so when you do switch your google rank and search engine searches to not plummet to ZERO. So this blog use to be btrbb.blogspot.com and I bout the domain btrbb.com. I then tweeted and shared my old posts over that 3 month period. I am not totally sure if that helped but I figured if I was inviting readers to my older posts on my new domain than maybe it would help the process go a bit faster.
If you are a new blogger or you only have a handful of posts on your blogspot address then you should probably just buy your domain on a paid word press hosting company and start over. I had 2 years into my blog so there was no way I was going to not get my domain before hand.
Next post will be how to create you blog before you transfer you domain over. Of course you will want time to make it pretty before you merge the blogger posts over.
Have a good one!