Strum your guitar with a pick or your fingers. Hold down the strings with your fingers in the appropriate shape and try to strum with your other hand. Acoustic guitar strings often have higher actions than electric guitars, so you may have to press down very hard to get a good sound. If the chord comes out muted, try holding down the strings with more force. If your string is buzzing, move your finger further away from the metal fret on your neck.
As a beginner, it is best to avoid making a pawn shop, flea market, or yard sale purchase unless you are shopping with somebody who has experience in purchasing a guitar. Having never purchased a guitar, you may not know what to look for in terms of damage or guitar quality. With that being said, a local music retail shop is your best bet. You may also shop online with a reputable company, although be sure to review the business’ return policy.