What to do will depend on what result you need. Why does it matter if a query "returns more than 1 row"? That's very common. If more than one row results, what are you wanting to happen?
Showing a few examples from the tables, along with the result that you want, will help us know how to respond.
Post the code you have tried so we can get a better understanding of the tables and columns in question. If you are looking for just one returned row, is it solely on the join column? If so then use DISTINCT like Splat mentioned, If it by more that one column then you need the GROUP BY. Without seeing what you have this is my best guess.
Post your code and we can show you how to correct it. It is not a difficult task. We are glad to help you figure out your issue but trying it helps reinforce the knowledge.
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