The data in Gnumeric are stored in the cells of the spreadsheet, each of which has a cell format which dictates how the data will be displayed, whether the cell will have borders and other information. Cell formatting can be quite confusing at first because it combines simple changes, such as the colour of the characters being displayed, with more complex ideas, such as how future changes to the cell will be interpreted.
All of the cell formatting commands can be reached through a context menu by right clicking on a cell and selecting the Format Cells... menu entry. This will open a dialog window with tabs which group together similar types of formatting. Clicking on the Font tab allows you to change the font family, style, size and colour. For instance, if the cell B2 contained the text "Hello, this is my first spreadsheet" then you can make this text bigger by selecting a larger font size.