Check out this video that might help you.

Even at 18 you may have some skills that you have learned like sales, marketing, research, data analysis, and others. 

However, I recently discovered that I'm missing about one-third of the rental income, and no explanation was provided. ( Actually, I just checked my account to get money: 10/2 $500, 10/17$700, 10/28$500).Originally, I understood that the management fee would be 10% of the rental income.

., but you’ll want to check on tax implications in your home country.Consider consulting a tax advisor specializing in U.S.

I posted on FB Marketplace to check demand and was able to find people interested in paying $1200 for 1800 square foot garage.  

I would recommend checking out the BP calculators and basing your spreadsheet on that.

I need to check out how the water bill works. 

It's likely you will have to redo most of their work when you get it checked as you don't want an inferior product out there if you are selling or renting it.

Send a letter giving them your/pm's contact and a detailed explanation on how to pay the rent (Helpful with older residents if you are going from check to online billing.

Spell out the complete agreement of how expenses, profits (or losses), and other responsibilities are to be split—put everything in writing.Do a background check on the partner to confirm there are no outstanding liens, judgments, or active lawsuits against them.Hope these suggestions help.