As people above have said, don't attack anything until the game tells you that the war has started, which, as also has been mentioned, isn't until September. So you've got a whole month of "practice". In the current career, I did just that. I sailed out on August 1, went to my designated patrol area and held drills. Track ships, practice calculating the intercept path, practice torpedo runs, etc. Also did crash dives to see how fast the crew got the boat under. Tested to see how deep I could dive safely.
It may seem useless to go out like that, but for me it was valuable practice. Also, if you're going to play the game for real, as a commander of that time, you don't have the knowledge that the war will start in 30 days. You're just following orders, as if being sent on a training exercise. There's tension in the air, sure, but no one in the Kriegsmarine at your level knew that in one month time, there war would be upon them.
Or, you could just sail into Scapa Flow, shout "Hey Tommy, look at this!", drop your pants and moon the Brits!
