Talk to people in your neighborhood or closeby about what good realtors they have encountered. Ask them what they like or dislike in a realtor. Gather some names. Then call and ask for meetings. Take notes of what they say you should do to get your house up to snuff in order to sell. Then ask them for "comparables". This is what realtors put together to figure out what to sell your home for. As you know, a house is really only worth what someone is willing to pay for it, so they look at comparable house sales in comparable neighborhoods. Comparable square footage, comparable real estate, etc. Don't sign ANYTHING until you CHOOSE a realtor. This is really important.
The comparables are what you need, rather than an appraisal. You CAN pay for an appraisal, but that's traditionally what the BUYER pays for after putting a deposit down on a house; they would put a contingency on their offer on your house appraising for enough so that they can qualify for the mortgage. If you got an appraisal on the house you want to sell, the buyer wouldn't accept your appraisal anyway, to be honest. They will pay for their own.
The best way to learn this "system" is to interview AT LEAST three real estate brokers. They will tell you what to do. If there are differences between what they say, ask why. It's okay to ask questions. It's smart to ask questions. If they pass you off, you know you don't want that realtor. You should have at least 3 comparables to look at.
Try not to get "hooked" by a realtor who pushes for you to sell for a big price - that's a bad idea in this housing market. You could end up with your house on the market for a long time, and have to drop and drop and drop the price because you listed it for too much at the beginning. Try to be realistic. Also, DON'T buy a house without your own contingency to sell your house first. Two house payments is killer!
I've never been a realtor, but I've bought and sold many houses, moving around with my husband for his various job changes. I hope this helps!