There are lots and lots of different things you can write about, but it does depend on the reasons why you want to write.
Are you writing as part of the role in your current job role, or are you wanting to write as a career?
If you are writing as part of your current job role, such as e-mail or letter, then you will probably know what you have to write about, but that does not stop you from embarking on a different writing role.
There are lots and lots of different companies wanting individuals to write for them.
There are also lots of individuals that are wanting help from people to write for them.
But what are people wanting done?
Well some of the companies will be looking for good quality content written for their business websites, others might be looking for blogs writing to keep customers up to date with any new interesting information.
A student might want help with his advertisement to earn a bit of money teaching someone to play the guitar, the young woman needs help writing her Curriculum Vitae.
There are lots of people out there, that are wanting people to write things for them, and they are also lots of different areas that require writing.
There are lots of magazines of nearly every type of genre, which might have availability for articles, especially if it is something that you have a personal interest in.
If you have got an interest, such as knitting, sewing, or something else, there will be many other people that are looking for ideas, setting up a website sharing ideas with other like-minded people.
Contacting magazine companies, most take comments, on something that you have seen written and it might get published, ask if they take articles you have written on the subject or ideas you have, they can only so no, but until you ask, you will never know.
In the meantime while you are still trying to break unto writing if that what you want to do, write, write, write, as much as you can, about whatever you like.