If the bike was not well cared for, the chain IMHO is suspect. KMC chains are not terribly expensive on Amazon and since you indicate that your bike is possibly not a high end bike Wal-Mart sells bicycle chains that will work. You will likely want a chain tool as you will probably have to remove a few links to get it to the length you need.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke