To insert a chart for your spreadsheet data, you can highlight the wanted cells and select menu Insert|Chart.... The new chart will be put somewhere on the same sheet of the data.
If you click on the chart, its menu will be shown.
By selecting its menu Chart|Move to own sheet..., you can move the chart to another sheet. The chart will have its own sheet and it will fill the whole new sheet.
This may be not what you want. Sometimes, you want a chart sheet which contains several charts representing data from other sheets. To do that, you can add a new sheet first by clicking the Add Sheet button at the bottom-left corner.
Select the new sheet. Insert a new chart by menu Insert|Chart.... In the Create Chart dialogue, you need to manually input what data you want to use. If the data are from Sheet1, you need to add Sheet1! right before the cells range.
Click Save chart button and you have a chart for the data from Sheet1. You can add more charts for the data from different sheets in this way, so that you can have a single "chart sheet" for the whole document.