I am guessing the target client would be SMBs? If so, what have you tried so far that either worked or didn't?
If I was at this task (disclosure: I do marketing consulting), I'd start by collecting intel on existing clients and those that you'd like to go after. Ask those that made the purchase why they did it and ask those that didn't sign up what was missing that made them look elsewhere. Study competition, try to understand what makes a software in this space go mainstream. Once you gain good insight, you start spending money on advertising on channels that cater to your target audience.