The commands on the Ribbon are the ones you use the most. Instead of showing every command all the time, Excel 2007 shows some commands when you may need them, in response to an action you take.
For example, if you don't have a chart in your worksheet, the commands to work with charts aren't necessary.
But after you create a chart, the Chart Tools appear, with three tabs: Design, Layout, and Format. On these tabs, you'll find the commands you need to work with the chart. The Ribbon responds to your action.
Use the Design tab to change the chart type or to move the chart location; the Layout tab to change chart titles or other chart elements; and the Format tab to add fill colors or to change line styles. When you complete the chart, click outside the chart area. The Chart Tools go away. To get them back, click inside the chart. Then the tabs reappear.
So don't worry if you don't see all the commands you need at all times. Take the first steps. Then the commands you need will be at hand.