Hi Myrthe,
I can't say that I learned to blog from a course or a book, however, there are publications I am sure which can help you.
A good place to start is by simply reading other blogger sites. I would suggest you read Seth Godin's blog
http://sethgodin.typepad.com/ but there are many other experts you can read as well.
The style for blogging is less formal and brief than an article. A good place to start is to find a topic of interest to you such as 'how to do something' and then explain in your words how this can get done in a few steps.
I am a contributor on a VA blog called Business Darlings
www.BusinessDarlings.com and you can also read the posts we (myself and several other VAs from VAF) have posted there. We each have a section that we cover. See if there is a section such as social media or technology or marketing that is of interest to you and then blog about that topic.
What I find to be important about blogging is that what you write about should be geared toward the audience you want to reach. So if you want to reach accountants, your blog should be mainly about accounting.
So knowing who your audience will be and then writing for that audience is important. Providing solutions to questions that they have will also prove beneficial.
Blogging does not necessarily have to be in the form of writing either. You can have a vlog or a video blog and you can write a post one day and post a slideshow another day. You can mix up the media so it remains interesting.
It is also important to blog about what interests you. It can become tedious to blog about topics which are boring to you.
Some VAs have business blogs while others have personal blogs and others have both.
The key is to be consistent and offer fresh content as often as you can.
On April 1st we will run a blog challenge contest to win a free book and another prize. Testing the blogging waters here on VAF is a great way to get started so do look out for that post and enter the contest if you wish.
Janine