It all depends on how exactly you have the string you want to print (i.e. if you want to format it first, if you have it as an io.Reader, if you have it as a byte slice...). And there can be a lot more ways.
By default the logger flags are set to Ldate | Ltime. You can change the logger format to any of the following (from the golang log documentation):
Ldate = 1 << iota // the date in the local time zone: 2009/01/23
Ltime // the time in the local time zone: 01:23:23
Lmicroseconds // microsecond resolution: 01:23:23.123123. assumes Ltime.
Llongfile // full file name and line number: /a/b/c/d.go:23
Lshortfile // final file name element and line number: d.go:23. overrides Llongfile
LUTC // if Ldate or Ltime is set, use UTC rather than the local time zone
LstdFlags = Ldate | Ltime // initial values for the standard logger
For example, flags Ldate | Ltime (or LstdFlags) produce,
2009/01/23 01:23:23 message
While flags Ldate | Ltime | Lmicroseconds | Llongfile produce,