Hello,
in the first mission the ships that are mentioned in the F5 view are target ships, so you can and should sink them. There is also a small "convoy" for training your targeting against moving ships and develop your skills. I definitely needed it with OneLifeCrisis' OLCE GUI mod lol.
You can destroy any ship that bears the british (!) flag, anchored ones are the green dots, BUT:
Do not leave the "harbour", remain inward and do not use the channel access to the baltic sea.
When you go back to Koenigsberg, along Pillau, you are the enemy towards all other nations including your own then in this first mission, so you will be shot at and have to "teleport" back to base. Be closer to Koenigsberg than to Pillau to automatically dock at the right location.
I had some renown of -630 then for sinking neutrals, but it does not matter. The tonnage you sunk WILL show up in the career though.
You do not need any renown, your boat is well-equipped, and after your fourth or fifth mission or so your renown will build up pretty fast - if you are a capable Kaleun that is

.
ANother tip: Do not equip your boat directly after a mission, but only when you start a new one. As well do not change any equipment before or directly after your first mission - old SH3 bug here.
In the second mission just remain inward again and cruise around, no targets this time but remain in the AO sector until the mission orders turn green in the F8 view.
Third mission your new base will be Kiel, so again stay in AOxy and transfer to Kiel when the mission orders for patrolling have turned green. If you get near Danzig or Hela be aware there may be polish ships patrolling ... but look at the flags.
I encountered a problem then. In the third mission before entering Kiel using SH3 Commander with "start at sea" option enabled, my game would crash to desktop. When teleporting to Kiel BEFORE reaching the green dot of Kiel, i could dock etc., but the game would crash next time i tried to load it. Deleted career agin, fifth time now. I will take another flotilla next time
Greetings,
Catfish