How companies divide company shares between founders and investors?

1. How do companies divide their equity shares among the founders and investors? For example, consider two persons are building a company. Person1 came with the idea of an e-commerce company. Person1 is taking care of developing the entire website. Person2 is taking care of image editing (product images). Some investors also joined with them. My question is how do they divide the company equity shares (condition is person1 should hold more than 50% of shares)?
2. How do other companies sell their shares in share market? Does it affect equity shares? How do companies release their shares to the market? For example, Company A released 100,000 shares, how are these shares valued? Is it possible to release such amount of shares to the market by a startup company to take company to a better position?
3. What are the methods and conditions for making first round of investment? Is it on the basis of equity shares? When should the first round of investment take place after starting the company?

I'm looking for a detailed answer or reference to any article that already answered this (I didn't find any good articles till now) and your valuable suggestions.