I couldn't understand why their credit wasn't better considering their income was what it was and their debts were not unreasonable.

As for the investing, I’ll probably be looking for a 3-4 family rental property to generate a little bit of passive income.   

Correct - Track all your income and expenses.The bookkeeping will determine whether you are profitable or not.it will also be good to compare from year to year your income / expenses to see what expenses you can potentially try to limit.I.E.

Consider duplexes or triplexes that can help offset your mortgage and provide steady income while you figure things out.

Same income, same credit, DTI ratio, underwriting requirements.

I am currently enjoying my 1k monthly income with no debt, and I am nervous about leveraging.  

I will definitely keep myself on here to stay updated and will definitely be interested in coming to the next one!

The calculator estimates that operating expenses are equal to 50% of the monthly income.  50% is a rough guide. 

Which is why I pay more, for less cash flow in highly desirable areas.In high income areas, rent is a lower proportion of income so therefore your tenants have a higher capacity for paying higher rent because rent is less than 20% or even 10% of their income.

Also, focus on 2 years of job/income stability.Class D Properties:Cashflow vs Appreciation: Typically, all cashflow with little, maybe even negative, relative rent & value appreciationVacancy Est: 20%+ should be used to cover nonpayment, evictions & damages.Tenant Pool: majority will have FICO scores under 560 (almost 30% probability of default), little to no good tradelines, lots of collections & chargeoffs, recent evictions.