Welcome aboard Jonah! For starters, look over the sticky posts at the top of the forum, they have tutorials for everything in the game.
General stuff:
http://www.subsim.com/radioroom/showthread.php?t=107783
Fire control guides:
http://www.subsim.com/radioroom/showthread.php?t=118923
A lot of good stuff there, and if you need more help feel free to drop in with questions.
As for evading escorts, you'll want to put yourself ahead of the convoy's path as best you can, go to silent running and low speed (~2 knots) and let the enemy come up on you. Then you get yourself into firing position and let 'em have it. After that you go deep for evasion. Keep in mind that the crew won't repair damage or reload torps when silent running is activated.
Hope that helps.