You can use any whistle signal for the commands as long as they're always the same, consistency is the most important thing. For my lot I use two pips for recall, one long (low) blast for stop and one pip for turning. If you have a good local gundog trainer (recommendation is usually the best way of finding a good trainer) then it could be worth giving them a call for a chat but to start with all you'll be working on is basic obedience etc. I always introduce the whistle straight away and start doing simple fetching games and stuff but 'proper' gundog training comes later. All dogs are different though and some are ready for more advanced training at a younger age than others, the key is to treat each dog as an individual and not to rush them or think that they should be at the same stage as another dog of the same age as that isn't necessarily the case.
I got Rodaidh at 6 months and all he could do was sit and walk on a loose lead so I started him straight away on the whistle for recall, proper heelwork, other basic commands like lie down, basic retrieving and then when he was 7-8 months we went on to off lead walking, the stop whistle, dropping to shot, quartering, distance commands, harder retrieves etc. and he passed his first gundog club test at 11 months but I've been really lucky with him in that he's incredibly biddable and very easy to train, Alfie on the other hand wasn't ready to do some of that stuff until he was well over a year old and he's never going to be as naturally obedient as Rodaidh.
