The required agreement when investing money in a company

I have been asked to invest money with a company to purchase goods, and then the profits from the sales will be split.

I want though to have an agreement in place that covers both parties as well as ensures all legal steps required are covered.

Money invested will be transferred by the banking system, and initial investment and profits from the sale will then be transferred using the banking system.

What is the right agreement, what will it look like, and how will I go about organizing the same?