The obvious thing to do is to use as many of the help settings as possible while you get a feel for the process of positioning your sub correctly for an attack. You'll find this isn't as simple as it seems, because your sub is not very fast unless surfaced, and even then it is only a little quicker than merchant shipping and certainly cannot outpace surface warships. So you have to detect things, get out of range and then skirt around them when you have figured out their general course, in order to position yourself ahead of them.
It's important to note that you will not hear things easily on your hydrophones unless stationary, so if you submerge to use them while doing this kind of thing and you want to make sure you are still in contact with them, come to a halt when you listen!
Setting up an attack like this is called 'doing an end around' and it is a typical U-Boat tactic. It's also where good skill in using the hydrophones comes into play while you are in position waiting for the enemy ships to arrive. Typically, you will start by detecting the enemy, figuring out his course, and then running parallel to it, slightly out of sight while you race ahead of the enemy. When you are ahead by a good distance (round about ten or twenty miles), you turn in towards the enemy's predicted course, submerge, and wait for it to cross your bow with your engines stopped, use the map drawing tools to plot the predicted line of the convoy and also to ensure that you are a nice distance from it for your torpedo shots. You should ideally be firing when the ships appear to be at between about a 30- 45 degree angle from you, so that when the torpedoes intercept the target, the target will be broadside on to you and present the biggest area for your fish to hit (later you might start to bend the rules a bit, but for now, this is a good tactic).
Keep in mind that torpedoes don't arm immediately when fired, they have to travel a couple of hundred yards before they will arm (although on easy settings in SH3, this is not an issue), so don't get too close to the line and don't get too far away either, as the further away you are, the more chance there is for an error to cause a miss. Opinions vary, but anywhere between about 800 yards and 3000 yards is going to give you a fair chance, although personally I tend to go for about 1000 yards if I can manage it, but that's a little close if there are destroyers escorting the ships, in which case I'd go for longer ranged shots.
Note that there is a good chance you won't miss on easy settings, as the crew will be making all of the calculations for the firing solution on easier settings, so all you will really be doing is 'directing the attack', but you can still develop good habits for later on when you are doing more of the work, so keep that in mind. When you are confident in your abilities to manage all that, you can start turning the help off.
Doing attacks this way gives you plenty of time to set up your torpedoes for an attack and get everything just right, in addition to which, all the targets should pass sideways-on to your bow, so your torpedoes will have a good broad area to hit and a better chance of doing so. If you can manage it, try and aim for ships which have other ships in the convoy slightly more distant and a little to their rear, that way if you miss, there's still a chance your torpedoes might hit something else.
Of course if there is no escort and you have a deck gun, use that, it's 'cheaper' and more fun, but bear in mind that even some innocent-looking freighters can be armed, and they might get a lucky shot!
Also bear in mind that if you are detected, or if you hit a ship, any escorts which might be accompanying the targets are going to suddenly get real interested in making life hard for you. So also have an idea of what you will do to get away. Don't get greedy and try for too many targets, you can always sneak away and re-acquire the convoy on the hydrophone and then set up another attack, and of course, you will already know where to look, because you'll know the ships will eventually resume their original course.
Typical evasion tactics for submarines are to 'sprint and drift'. i.e., you run quickly away from any oncoming destroyers, and then go silent by reducing your speed and ordering 'silent running'. your sub takes a while to slow down, and you can use this speed to turn quickly and then creep away at low speed from the location the enemy thinks you are at. Deeper is usually better, but there are any number of tactics you can adopt, changing depths is also a good tactic, but remember that you'll dive quicker than you rise, so plan your moves carefully. Unusual tactics such as hiding under or near a crippled ship to stop a destroyer from passing over you, or sneaking under a convoy and attacking it from the other side while the destroyers search in the wrong place are all valid things to try, and that's all part of the fun - it's more of an art than a science in many respects.
When you've sussed all that out, it's then time to start thinking about being more cunning with your weapon's and boat's capabilities; some torpedoes are more easily spotted by enemies than others, some have better speeds or longer ranges, some subs turn better than others and can dive deeper. Do you attack at night or in the daytime, are enemy planes going to show up? These are all things to consider, as is where you are when you attack, if the water is shallow, it removes some of your opttions for escape for instance. Knowing all this and planning things in advance and knowing your enemy's capabilities is what will help you to succeed and is something to look forward to when you have the basics figured out.
Above all of that, the main thing is to have fun and play it the way you enjoy it.
Chock