I'm cofounder of a software company in Australia. We are gaining traction internationally, particularly North America. We sell our software in US$ with approx half our sales via credit card and half invoiced. Looking to put on a US based sales manager. Sending money back and forth from US to Australia is a pain and costing us in exchange fees.
The best way to incorporate a software company in the US is to request a consultation with a US-based attorney. Start by seeking an attorney in the state where you plan to hire your US-based sales manager.
Answering this question in full without knowing a lot of business details will lead to an inaccurate answer for your company's specific needs. Therefore, a consultation is best to obtain a full response. During the consultation you will be asked about the existing business, the targeted states for transacting business, any proposed location for your sales manager(s) and other business activities in the US. Other variables involved in answering your question include tax implications, existing and estimated revenue and gross profits, estimated employees over the next year in the US, and a slew of other details.
Once an incorporation state is elected, an attorney can assist you elect a business entity type and ownership structure most beneficial to your business needs.