Writing a program that divide numbers until I hit a one digit number

One way might be

NestWhileList[Floor[#/2] &, 100, Length[IntegerDigits[Floor[#]]] > 1 &]

(* {100, 50, 25, 12, 6} *)

NestWhileList[Floor[#/2] &, 500,Length[IntegerDigits[Floor[#]]] > 1 &]

(* {500, 250, 125, 62, 31, 15, 7} *)

Or if you prefer to code it yourself

foo[n_Integer, k_Integer] := Module[{z},
   If[k == 0, Return["Error k=0", Module]];
   If[n == 0, Print[0]; Return[Null, Module]];
   If[k == 1, Print[n]; Return[Null, Module]];
   If[Abs[n] < Abs[k], Print[n]; Return[Null, Module]];
   If[n == k, Print[1]; Return[Null, Module]];
   Print[n];
   z = n/k;

   If[Length[IntegerDigits[Floor[z]]] == 1,
    Print[Floor[z]];
    Return[Null, Module]
    ,
    foo[Floor[z], k]
    ];
   ];

   foo[100, 2]

   foo[500, 2]

You can also use

FixedPointList

ClearAll[quotients1]

quotients1[n_, k_] := Most @ 
   FixedPointList[If[IntegerLength[#] > 1, Quotient[#, k], #] &, n]

Examples:

quotients1[100, 2]

 {100, 50, 25, 12, 6}

quotients1[950, 3]

 {950, 316, 105, 35, 11, 3}

ReplaceRepeated

ClearAll[quotients2]

quotients2[n_, k_] := {n} //. 
   {a___, b_} /; IntegerLength[b] > 1 :> {a, b,  Quotient[b, k]}

Examples:

quotients2[100, 2]

 {100, 50, 25, 12, 6}

quotients2[950, 3]

 {950, 316, 105, 35, 11, 3}