Hey NoX Welcome to Subsim first of all
No to answer some of your questions you need to understand that being a submarine/uboat captain is a constantly evolving battle. You always learn new tactics and learn new tips and tricks.
1:Patrolling an area is, just what it sounds like, patrolling an area. In general it consists of alot of time spend driving searches through an area (I generally select 50km around 'point' or objective my Flotilla guy gave me).
To find that out, go to your Navigation Map (top right corner, the wheel icon). No, find where your sub is and where you want to go. To manually get there, you can just select a heading with the compass (provided you have a Mod that lets you use the compass..) OR you can click your sub and drive yourself there.
Now, select the Compass tool on the left side of the map, click on the objective point or as close to it as you can. Now drag it out so it's the size you want, 50km in this case.
Now, plot a course around and around (spirals are good) and search the area. If you have TDWs Mod enabled you'll have access to shipping charts which will show you the best areas to patrol (not 'best' per se, but just where more merchants are bound to go).
As a rule (for me anyways) I only stay in one 50km area for only 72 hours. After that I find a new area and start again. Note though that you're going to want to periodically stop and dive to a depth lower then 12 Meters to check your hydrophones.
2: The map can be confusing, and it took me a little while to get confortable with the radio contacts. When you're looking at the Map and you see all these red icons (squares [merchants], diamonds [warships], triangles [planes], and circles [submarines]) you'll notice that some are far, and others are close.
If you see an icon appear close to you (square preferably) then click your sub and plot a course along its direction of travel. As you're dragging your plotter along, you'll notice that the red square is moving. This means that, if you stay at the speed/heading you're at, and he stays at the speed/heading he's at, when you reach the end of your plot line, you'll both be in the general area that the map shows. To utilise this effectivly, plot a course so that, when you reach the end of the course, you're both fairly close. This will help you find merchants alot easier.
3:When you see 6-8 or more ships sailing together, this is a convoy. This is a defence measure against you. Now, sometimes you'll find that there are no warships around, this is good. Other times (especially late in the war) there will be more and more destroyers escorting the convoys. To use the number system in the game, you basically want to align the dots. preferably the 1 or the 2s. So on your map, you select a torpedo speed and rotate your periscope so that on yur map the 1 on the line extending from your sub matches the 1 extending from the ship your attacking. Same for the 2 or 3.
As for the other 'auto' TDC, make sure your TDC is OFF. Lock onto a ship (crosshair over the ship and SPACE). No it will say tracking new target. UNLOCK the target and now when you put the crosshair over the target, it will computer everything for you.Point your scope where you want the torpedos to hit and fire.
Please note, if your closer then 400m the torpedos will not arm themselves and you have effectivly blown your cover. Because your torpedos dont arm themselves, they dont blow up. So make sure to attack from 500M+ (inside of 1000M is best).
4:When you surface close to a merchant ship, hop onto the bridge and go to your binoculars. Aim them at the ship and push space and you should see a little triangle appear under the ship. This means your crew is targetting them with the deck gun. They will take care of the rest. Please note this does not work with the UZO (the binoculars that are centered on the bridge, but just the ones you 'carry').
5: I would love to give you some tips and tricks but seeing how this post is really long just PM me on this forum for any help or questions you need. To attack 5-6 ships simultaneously is difficult but possible. Just be patient with the auto TDC and lock them up. Using manual is harder but alot more rewarding
Just take your time and enable options that you think will help you get the basics down
Also, there are submarines in the game (enemy and friendly) just hard to find is all
Cheers, and good Hunting !
Krauter