When mine needed babysitters I asked friends to recommend them to me. I also used the oldest girl of 4 across the street. I watched her with her sisters and she was very good with them. So she became my sitter too.
The people who have had my daughters babysit have also been good friends of mine. I make sure I am home when my now 13 would sit. I do not have to be around any more when my now 15 yo sits.
Have a teen come over and interact with your child while you are at home. Have something to do, like clean upstairs and she and the kids play outside. THen you can check up on them and see how she is doing.
My son took the babysitting class and was the most wanted babysitter when he was 12-14 ish. He would get out and play baseball and really run the kids. MOms loved him.
Make sure you have a list of your cell, the house address, a neighbor, and another emergency contact, maybe Grandma or an Aunt.
Make sure you have babysitter food. Chips and soda, let them know what is OK for them to munch on.
Do you want them to do any picking up? Have a list of what needs to be accomplished, what time is bedtime, are they to serve dinner, what the kids like before bed.