If it’s a “one way” serialization (for debugging or logging or whatever) then fmt.Printf("%#v", var) is very nice. (Update: to put the output into a string instead of printing it, use str := fmt.Sprintf("%#v", var).
If size matters you can use %v, but I like %#v because it will also include the field names and the name of the struct type.
A third variation is %+v which will include the field names, but not the struct type.
They are all documented at the top of the fmt documentation.
If you need two-way serialization JSON, Gob or XML are the easiest/built-in options in Go, see the encoding packages.