I will be joining a new start-up as a technical partner. Other two partners are business administration role.
I will be the one who will program everything from scratch and build the new web based product.
Who will own the Intellectual Property Rights to the software code, methodologies and ideas, Copyright to the blog posts and marketing content written by me and new innovations done by me which is eligible for patent?
Who will own all these rights when I leave the company?
All founders, and later all employees, should sign a binding agreement to assign all copyrights, intellectual property, etc to the company.
This is standard practice and protects the company in the event that someone leaves.
Standard disclaimers apply: IANAL, IMHO, YMMV.
Who has a claim to the intellectual property depends on the nature of the business relationship, the jurisdiction where you are located, and any contracts that may contain provisions governing your rights--such as employment, non-disclosure, and assignment agreements.
First, is your business properly incorporated? A partnership? Or do you work with each other on a contract basis? The more formal the business arrangement, the more likely the business owns the invention or copyright or both.
Second, did you use company equipment, confidential information, or know-how to make the software? Did you do the work during hours you work at the company? Is the software related to the business of the company?
Third, who is listed as inventor(s) on any patent application? Absent an assignment (by contract or operation of law), inventors are presumed to own equal undivided interests in the resulting patent.
Since much of this depends on the jurisdiction in which you and your company are located, the best approach would be to speak with an attorney.
Last, it probably would benefit you to have a written agreement with your partners/co-owners/employer to address this. Good luck.