Let's measure the size of your desk using a meter stick. Most meter sticks have spacing of 1 mm. Thus, the closest you can measure is to the nearest single millimeter, and then you can estimate to a 1/10th of a millimeter (if you're good!). For discussion sake, the "zero" end of the meter stick is broken off (Uh, oh!).
No worries.

Follow this simple example:

To find the width of your desk, place the meter stick along the front of the desk, letting the broken end extend beyond the left edge. This forces you to choose a starting number (because "zero" is not available). Read the meter stick at this point, say:

W_left = 152 mm.
You read the right dimension as W_right = 816 mm.
Obviously,
W_desk = W_right - W_left
W_desk = 816 mm - 152 mm
W_desk = 664 mm
Repeat to verify your numbers recording each trial.
They may not be the same each time!
Similarly, you find the length to be L_desk = 561 mm
This trivial case demonstrates each facet of taking measurements using:

• Fundamental units - length
• Measurement system - SI (also called metric)
• Instrument - meter stick
• Calibration - Find the "zero" point, W_left = 152 mm.
• Taking measurements and recording the numbers - log book/data table (You did write down the data, didn't you?)
• Errors - Repeat until confident of numbers.