Welcome to the boat mate
There is a ton of information in this thread.
http://www.subsim.com/radioroom/showthread.php?t=146795
Some of it won't make much scents till you get some experience but it will come.
I wouldn't worry too much about the fourth training mission, you start out in such a bad position it's all but impossible to complete. It's more a DD evasion mission than a sink ships thing.
Best thing for training is the single war patrols like hellcat, got radar and a better boat. Don't worry about the mission just go to the Luzon Strait, find some targets, save the game as soon as you do and replay it till you get a good handle on how things work.
Got questions? We got answers. Ask away.
Magic