Append a folder name to a path variable in applescript?

set fold to "/Users/Test/Desktop/"
set file to "/Myfolder/Hi.txt"

How can I join these two variables to get my file path as "/Users/Test/Desktop//Myfolder/Hi.txt"?

here ya go

  set aFile to (the POSIX path of (path to desktop)) & "Myfolder/Hi.txt"

Use the Concatenation operator to join strings:

set the folderPath to "/Users/Test/Desktop/"
set the filePath to "/Myfolder/Hi.txt"
set the fullPath to folderPath & filePath