To select a command, you can go to anyone of the various options available on the ribbon/toolbar or just type the command on the keyboard. Let us go through a few of the common commands that are used in AutoCAD and their keyword (these can be found when you hover the cursor over the icons on the ribbon)
Line command selected; The number are the coordinates (x,y) of the point on which the cursor is on.
|LINE||LINE/L||Creates a line with twopoints and a dimension|
|CIRCLE||CIRCLE/C||Creates a circle with aspecified radius|
|RECTANGLE||RECTANGLE/REC||Creates a rectangle with 2 dimensions of length andbreadth|
|POLYLINE||PLINE/PL||Polyline is a line with morethan 2 points, each line tangent to previous|
|HATCH||HATCH/H||To hatch or fill an object area with the desired pattern type|
|POLYGON||POLYGON||Creates a polygon with a minimum of 3 and max1024 sides|
Once you have started a command, you will see a plus shaped cursor, which helps specify the first point.
There are also numbers their which keep changing as you move it, these are the x,y coordinates, this is a 2d plane so there is no z coordinate.
Start the drawing by clicking with the left mouse button (LMB) at any point and then moving your pointer to the next point and pressing LMB again.
This will create a line in case of Line command, a circle with the specified radius in case of a circle.
With rectangle, the two points can specify two opposite corners which can be joined by diagonal of the rectangle.
Alternatively, you can enter a dimension after specifying the first point in the direction you are taking the line and press Enter. In AutoCAD, the command continues until you press ESC, and you can keep on specifying next points.
Also, in line command, you can press tab to set an angle to the line after specifying the first point or dimension and then set the value