Welcome to Mathematica!
an introduction to version 3.0
You have just opened a Mathematica "notebook." This one is designed to give an introduction to some of the many features of Mathematica 3.0 - the mathematics as well as the workings of the program. Many of these will be seen again in more detail in later notebooks.
What you see now is similar to an outline form, with section headings shown. Below this paragraph, find the line with "Numbers" in bold print. ("Numbers" is a "section" heading.) Move the cursor to the far right of that line to the vertical bar with the little triangle at the bottom. The cursor will change to a vertical segment with an arrow. Double click on this bar with the triangle to open the section. Try opening and closing the section a couple of times. Then follow the directions.