@Hannah Leasure , the calculators on here are a great start. I use them for every deal that I look at. The reports they produce will help you understand the ROI.
Hope this helps,
I would second what @Bob Langworthy said. Also, in the meantime I would HIGHLY recommend you check out some of the local meetups. There are some great ones in Harrisburg, Camp Hill, York, Lancaster, etc. Download the 'meetup' app and search real estate and all the local one should show up. Also, start walking properties too so that you can get a feel for things and that will help you pick a more accurate number for cap ex/repairs/rents and then when you start using the calculators you have a pretty good idea what things will rent for in certain areas and helps you understand your market. If you're not sure what you're looking at take a contractor or someone that does.
For me, I use 8% for vacancy (8%= 1/12 = 1 month of vacancy and I always do better than this) and 10% for management (I self manage), and anywhere from 5-10% for cap ex and 5-10% for repairs depending on age/condition/etc of property. This is after factoring in any other utilities/insurance/taxes. These are rather conservative numbers, but if I can hit 100-150 a door a full asking price I'm looking much closer at the deal.
Let me know if you have any further questions or I can be of help in anyway. I'm very familiar with Harrisburg and many surrounding areas.