Hello, I would say choose a specific subject which you can write about for a long time. Don't make the blog too general, if you have multiple ideas, write multiple blogs but bear in mind they will take time to maintain.
Blogging is hard work, be prepared for a long period of little recognition, they can take a year or two to grow to high traffic levels, but if they don't be prepared to cut your losses and start again.
A good way to promote your blog is to register with Technorati.com. They will add the blog to their catalog and it can boost your traffic quite easily.
Also promote your blog with an RSS feed to your hubs and promote your hubs with links from your blogs.
Despite all this. Have fun. That way you'll be able to build a successful blog.
All the best.