Hi Linda, thanks for posting! Full disclosure - I run a property management company as part of my team, but here is some general guidance that should help you get started.
Here in the Lehigh Valley there are two main things every management company will charge. The management fee is a percentage of collected rents, and most new investors should expect to pay 10%. This covers all administration, accounting, rent collection, and maintenance. As you build a larger portfolio, you may be able to negotiate this down. The next is your tenant placement fee. Pretty much all companies here charge one months rent for this, which includes turning over the unit, marketing, showing it to tenants, collecting applications, background checks, lease signing, and move in. Each company will have their own other costs, to include lease renewal fees, maintenance fees, setup fees, and eviction / legal fees.
PLEASE - When you sign your management agreement, ensure that there is a maintenance threshold specified! This is a specific dollar amount that gives the PM authority to make repairs up to that point, but requires your approval for larger expenses. I had a PM company charge me $1,000 to chase a squirrel out of a garage. They had a pest control specialist drive up from Philly instead of using someone local! Because this maintenance threshold was left blank on my management contract, I had no recourse. (Side note: That's the story of why I started my PM company)
If you have other, more specific questions, please post them! I'd be happy to help with any answers you need.