how would i invest in an already existing business

If you are talking about an investment (where you share in the profits and losses of the business) as opposed to a loan (where you loan the business money and get repaid, with interest, but get nothing extra if the business does especially well and are still repaid if the business loses money) you can pretty much structure it however you want. What is the current structure of the business?  Is it a corporation?  If so, the owner just has to sell you some stock.  If you are supposed to own 25% of the business in exchange for your $5,000, he would sell you 25% of the stock for that $5,000.  A similar method would work if the business was an llc.  If the business is a sole proprietorship (that is, it is not a separate legal entity like a corporation or llc, but is just owned and operated by an individual), you could structure it in other ways.  For example, you could contract for you to invest $5,000 in exchange for a percentage of net profits (which you should carefully define so you each know what revenues you are to share in and what expenses are to be deducted "off the top"; although you used the term "revenues" in your email, revenues don't mean that a business has any profits to share with investors.) As for how and when you get paid, that's up to the two of you to agree on.  There are a lot of issues that the two of you should agree on as well.  For example, are you going to be working in this business?  If so, what is your salary going to be?  Is the current owner going to get a salary for working in the business and, if so, should this amount  be deducted from revenue for purposes of calculating the profits to which you are entitled to share?  Who calculates the net profits in which you are entitled to share?  Suppose the other person disagrees with the calculation?  Does that person have the right to audit the books?  If so, who pays for the audit?  Do you both get a say as to how the business should be run?  If so, how will you resolve any disagreements?  What about any profits and/or losses which accrue before you invest - do you share in them?  How about creditors of the business existing before you invest - do payments to them come out of your profits?  If the business is not a separate legal entity, how will you shield each other from claims made by  your individual creditors, having nothing to do with the business?  Etc, etc.