On the Drawing toolbar (at the bottom of the PPT window by default), click
on the line tool.

Draw your long straight line (the axis). Hold shift while you drag to create
the line so it stays straight.

Draw the tickmarks.

Select one tickmark and the axis line and choose Draw|Align or Distribute,
and click the button to align the left sides.

Then select another tickmark and drag it over so it is at the right-side end
of the axis line.

Make sure all the other tickmarks are between these two tickmarks on the far
left and right of the axis line. Then select all the tickmarks and
Draw|Align or Distribute, and click the horizontal distribution option. Do
not tick the "relative to slide" option.

Then select all the tickmarks and the axis line and Draw|Align or
Distribute, choose to align the tops.

The grid can be determined by using "View", "Grid and Guides...", "Grid
settings", "Spacing:" You can choose the distance between those. While
here you may want to look at Snap to and see if "Snap objects to grid"
is checked plus "Grid settings", "Display grid on screen" is checked,
then "OK".
Create the axis using "Line" from the tool bar for the x and/or y axis.
Then make perpendicular marks on the respective axis that you have
established as x or y.
Graph On!

Echo's method is good. Hers another way for completeness.

Draw the line. Add the first tick mark. Press ctrl + D to get a dupe and
then move the dupe to the correct position for the next tick. Now press ctrl
+ D 8 times , Hey, auto tick spacing!

If its not quite right you can resize the line slightly.