If you are serious about blogging, if you intend to use a blog in any commercial way and/or to create a professional reputation, then whether or not you decide to have a free hosted blog at WordPress.com or hire a web host and self host a WordPress.org blog, I recommend purchasing a domain as early as possible. This is because blogs on subdomains like wordpress.com lack the credibility that those on their own domains have, and a domain name tends to give your website an aura of respectability. Having your own domain means provides increased opportunities to:
build your own unique brand, online presence, and reputation; assist your followers (readers, clients and customers) to recall and locate your site very easily.
The pros and cons of being free hosted by wordpress.com or self hosting wordpress.org software are summed up very well in WordPress.com vs. WordPress.org. http://support.wordpress.com/com-vs-org/
If you are an experienced WordPress.com software user, and have the skill sets required to set up and manage your own self hosted WordPress.org install then the instructions for making the move are easy to follow. But if you are not skilled at WordPress.com blogging, and also lack the skill sets required to self host your own WordPress.org install then acquiring those skills first is recommended.
Here at WordPress.com Staff does for us what we must do ourselves on a WordPress.org install. When you have a WordPress.org install you are responsible for all upgrades, installations, backups and troubleshooting. If you install a theme or a plugin that negatively affects your blog, then you have to figure out what went wrong and fix it. Sometimes it is a conflict with another plugin, sometimes is it is just a poorly written plugin. For self-hosting figure an average monthly total cost (including cost of a domain name) of about $10 to $25 generally, but that all depends on storage and bandwidth requirements.