First of all, welcome to the world of Harpoon. Good purchase you wont be sorry
Your submarine will launch its own decoys, but must change its course speed and depth yourself. I am not very good at explaning it, but the idea you need to keep in mind when you have torpedos coming your way is to put as much distance and angle between you and the incoming torpedo, so that when its seekers go active you are not where the launcher estimated you to be.
This problem will be complicated however if the launcher fires a spread of torpedos covering a variety of angles. I which case you need to figure out where to put your sub to try and avoid those seekers from seeing you when they go active.
Do not go above cruise speed if you can help it when you are engaged, if the torpedos are wire guided to their active point, the launcher can adjust the torpedos travel to your course, you need to remain as queit as possible and change depths often. If you are given the room to do so, complicate the firing solution by changing course often also.
Bear in mind if the launcher was a helicopter or an aircraft that has a good fix on your location and course, they will most likely drop a torpedo too close for you to evade, the probability of a kill for airborne platforms is very high with proper detection and a close shot.
This is not as much detail as you should know, but this should give you a basic understanding of what you need to do, to avoid those torpedos.
The red or orange wired box around contacts that appear on your display, is showing you the possible area that the contact is located in according to the information obtained by your craft and the crew, this is not always exact, but should give you a good idea about where that contact is. As the information recieved by your craft grows in accuracy and the crew works more on the possible solutions for the contact, the contact area frame will reduce in size and change shape to show increased accuracy.
It is best to fire when you can be sure of the contacts course and speed and that box is as small as possible, your firing solution will be much more accurate, and as a plus factor you can hazard a guess at where torpedos or missiles may be coming from.
Hope this helps
Good luck