The answer depends somewhat on why you need the information. If you hope to refinance, you'll need a formal appraisal to firm up what the value is. If you plan to sell, an appraisal may not be much use until there is an offer on the house. Zillow.com and other sites can give you a starting place, but they are just that, a place to start. The Zestimate may be as much as 10% high or low in many markets and depending on the condition of your home.
Since you indicate you are a seller, interview a few agents and ask them to provide a market analysis. If you are planning to sell without an agent, discuss this with the agents and listen to what they have to say. A good agent is worth more than what you will pay. You can also consult agent who helped you buy the home would love to chance to reconnect and can run an up to date CMA to give you a better idea.