Hi there and welcome to WordPress. You can write/blog about anything you like and whatever interests you. It doesn't have to be on one subject if you don't want to do that.
Have a look around the 'Reader' and the 'Freshly Pressed' sections (found on the home page) to give you an idea of the many subjects and topics covered in blogs.
A journal with photos might be a good idea to start with; family life, kids, pets, holidays etc and you'll find that, as you become more confident you will want to include other things in your blog. Short stories, poems, anecdotes, even videos.
So let your imagination guide you and remember, quality content will get people reading your fab new blog and come back for more.
Good luck and happy blogging.