In the Start tab of the charts dialog box, you can see recommended chart types. Based on your selection of data, Google spreadsheets chooses up to four chart types that will do the best job of visualizing your data, eliminating the struggle to figure out what type of chart is best. For the best recommendations, select the range of data you'd like visualized in your chart before selecting Insert > Chart.
You can see examples by exploring chart types.
If the recommended chart types don't match the chart that you envisioned for your data, you can always explore more chart options in the Charts tab or by clicking the More >> link next to the recommended charts.