A lot of it boils down to what you want to offer, and who you want to work with.
The wrong people will call you for the right service, and the right people will call you for the wrong service.
You can help cut down on those types of calls by researching who uses the types of services you want to offer, or what kinds of services your target market requests the most.
If you say, "I'd like to work with realtors," then your next question is 'why?' and your second question is, 'what kind of help do they need?' You then find out that they might need help with website updates, or listing flyers, or uploading photos, or sending out follow-emails and info packets.
You can specialize even within a specialty.
And don't be surprised if your specialties or services change over the course of the first year or two. Sometimes your market will mold you, other times you'll find that you really enjoy a service you didn't initially offer and make the move to that. A VA I know started in social media and ended up doing SEO
and training other VAs in social media instead.
You have to get started in order to fully find your calling. Take it slow, do your research, and take notes with every call and every client to see how people are viewing you and if it's in alignment with what you are setting out to do.