Do some research or ask friends/family that you trust for a real estate agent referral. Like mentioned below - an agent can do an analysis looking at similar homes sold in your area. When interviewing multiple agents remember the best realtor is not necessarily the one that tells you the highest price for your home. It's usually the most honest one that you believe will give you the best customer service.
Contact a local real estate agent and ask them to do a market analysis. They will look at comparable properties that have recently sold, and let you know what the home is worth. The link below explains a bit about valuation.
A Realtor who works the market will always be able to help you determine the value of your house for sale. We have access to sales figures and know the areas.
I work both in Galveston and Houston and would be happy to help you.