Figure how who your target market is, this is the number one starting point!
For example, I shoot new model portfolios, actor and business headshots, architecture & interiors, and marketing material for small businesses. Each of these is a different target audience and so I need to focus my marketing on each of them differently.
What do you enjoy shooting and what are you GOOD at shooting? Also knowing what you are not good at is just as important.
I know that weddings and events are not my thing after trying a few, I just don't enjoy shooting them. It's better to focus on what you are good at than try to take on everything.
I don't really make money from this site as most the models are working models (including myself so I understand) so knowing WHERE to advertise your service and network is important too.
Attend networking events suitable for the areas you want to get into, join Facebook groups, Linked In groups, generally promote your services on as many suitable platforms as you can.