First of all, you need to have a starting point: what is the issue your offer is trying to solve? Then, which kind of people or companies consider that issue a burning one and therefore will be ready to pay?
Once you have a clear definition of 3 or 4 targets, you will be able to double click on where they hang out, how they acquire info in order to solve the issue, how they phrase that issue...
Only then you can test with google or facebook ads and see if they click (that's a first win) and if they buy the product/service. If you don't have a product or service ready yet, you can ask them to leave their email for more info. that would be sufficient as a start.
I'm happy to jump in a call to help you define/finetune the issue you are trying to solve and 3 or 4 possible target audience. Serena