If you have ever set your font or font size in word processing, you are already equipped to change the font and size in Excel. First you would select the target cell(s) and then pick the font. The actual list of fonts you have at your disposal depend on which fonts are installed on your computer.
Just as in other programs that let you select Fonts, Excel provides a font field and a down arrow beside that field. If you click on the down arrow, you will see the list of fonts. Scroll down the list to find the font of your choice.
The size of the font is selected is a similar manner. Next to the font field is located the size field. Click on the down arrow and select the size you need for the cell(s) selected.
A word of caution about selecting fonts. The default font (from the factory) is Arial. This font is commonly installed on most computers. Should you decide to change the font to a "special" font called "SuperFont #5" you have installed on your computer, you make the change as described. However, if you plan to distribute electronically your Excel project, the people who try to open your project won't see the text in "SuperFont #5" because they don't have your custom font installed on their computers! If you plan to distribute your Excel project, try to stick to fonts that are common and all who view your project will see it just as you do on your computer.