When making a two-digit number,

you can choose the first digit any of 3 ways, 1, 2, or 3.

For each of those 3 ways to choose the first digit, there
are 4 ways to choose the second digit, either 0, 1, 2, or 3.

So that's 3×4 or 12 ways in all  Here are all 12 ways:

 1.  10
 2.  11
 3.  12
 4.  13
 5.  20
 6.  21
 7.  22
 8.  23
 9.  30
10.  31
11.  32
12.  33

Edwin