We are first going to write the backup package, of which we will become the first customer when we write the associated tools. The package will be responsible for deciding whether directories have changed and need backing up or not as well as actually performing the backup procedure.
One of the early things to think about when embarking on a new Go program is whether any interfaces stand out to you. We don't want to over-abstract or waste too much time upfront designing something that we know will change as we start to code, but that doesn't mean we shouldn't look for obvious concepts that are worth pulling out. If you're not sure, that is perfectly acceptable; you should write your code using concrete types and revisit potential abstractions after you have actually solved the problems.
However, since our code will archive files, the Archiver interface pops out as a candidate.
Create a new folder inside your GOPATH/src folder called backup...