The good news is that most of the sims already mentioned can be customized as to difficulty.
At first I was going to recommend some of the older sims, but not only are they harder to get, but let's face it, the graphics are better now.
If you are interested in WWII submarines, you can pick your side: SH3 for Germany and SH4 for the US. Each are fun. Set the difficulty to its lowest and just have fun. Always remember that the purpose of these games is to have fun. There are well respected people on this site that have been playing for years and still keep the difficulty low. The key is to pick the difficulty that keeps the game fun for you.
Then I can recommend many of the strategy posts in the SH3 and SH4 forums on this site. Many really smart people have taken a lot of their time to write tutorials and published them here and also on youtube. Just like in the real navy, we all learned from the old timers.
But don't rush into things. The last thing you want is turn a game that used to be fun into something that is frustrating. Don't worry, soon you will be doing manual targeting if that is your interest.
All these games can be changed with downloadable modifications or mods. The mods are in the Download portion of this and other sites. There is considerable argument whether new players should stick with stock games at first, or jump into mods. And both sides are right.
My opinion: You have plenty of time to mod up. You won't know which mods you might like until you experiment with the stock game for a little bit. Just because a particular mod is popular does not mean that "you" will like it.
Good luck with it. Subsims are not one of those games you will tire of after a few weeks. People here play the same subsim game for years.
Welcome to the board and welcome to submarine simulators.
Now quit posting and install one of the games and start sinking the floaty things!!