I am attempting to make a unit converter that converts between km, m, cm, miles, yards, feet, and inches. I have a textbox that is used to input the value to convert followed by a space and the units (i.e., 56 km) and a second textbox where the user types what unit they want to convert to (i.e., cm). When converting, the output will be displayed in the second textbox. (I know this is complicated, but this is the way it is to be done).
I am doing this using sub and function procedures and If blocks and Select Case blocks (i.e., keeping it as simple as possible).

I'm not sure how to declare the variables because they contain both numbers and letters so I declared as strings (Dim unitFrom, unitTo As String = txtUnitTo.Text, etc.). I then tried to use case blocks where:
Select Case unitTo
Case "cm"
txtUnitTo.Text = CStr(txtUnitFrom.Text) & "cm" [to display the number and unit -- i.e, 5,600,000 cm]
This, however, is not working and I think I am looking at this all wrong. I have also tried using the Double variable but am running into the same problem.

I also need to add a comma every 3 digits in the answer, which i am unsure on how to do. Any help would be appreciated on where to start or how to accomplish this. As an added note, I am a beginning programmer so if explanations could be kept in simple terms, I would appreciate it.