First you must run a valuation for your startup. Then, based on the results and the amount of capital you need, you will offer a percentage from your total shares for a certain amount of money. For example, if your startup is valued at $1 million and you need $100,000 to reach your next milestone, you will give away 10%.
This can also be translated in a # of shares. If your startup has 1,000,000 shares according to the valuation, and each share is $1, due to the need for $100,000 you will sell 100,000 shares.
In this case, we advise you to establish a rule that you will sell only in increments of 50 shares. By doing so each individual must invest a minimum of $50.