re: When the question is put to your bishop, will you die to take that knight?
There are a couple things to consider. First what if any tactical reason do you have for pinning the knight? In this game his knight was attacking your pawn on d5. His pawn on c4 was also attacking your pawn. With the knight unpinned, the knight could take the d5 pawn and be protected from your queen by the e4 pawn.
However, since you took whites c4 pawn with dxc4 it might have been better to have not pinned the knight in the first place and play dxc4 sooner. But since you had it pinned was probably just as well to take it as otherwise you'd be down a pawn and facing a protected knight on d5.
The second thing to consider is how taking the knight will change the long term situation. There are several pluses and minuses here and what to do depends on what your plans and playing style are.
1. Loss of bishop pair. Taking the knight means you lose one of your bishops. Two bishops are often considered superior to two knights or a bishop knight set, particularly in the end game. But if you aren't fond of playing with the bishop pair or don't expect to keep the other one around very long, this is no great minues.
2. Putting holes in the pawn structure. One important thing to think about is what piece is going to retake your bishop. If it is one of the pawns, this can be an important plus. Doubled pawns are hard to protect and easier to take later. If your opponent takes with the knight's pawn this could put a hole in front of a castled king which is almost always a good thing for you! Taking with the king or queen's pawn reduces the number of center pawns your opponenet can use to control the center, also a good thing. Of courese if the knight will be taken with a bishop or queen none of this helps you.
3. Weaken castle defense. The knight on f3 (or similar) is a very good defender of a castled king. Removing it can make the castled king more vulnerable to attack. However, if the other knight is in a position to replace the captured piece this is not terrible useful.
So to summarize, first thing to considered is the tactical implications of taking the knight. If there is some immediate benefit of taking it, or immediate loss of moving the bishop back best to take it. This was the case in your game, though you might have been better not pinning it to begin with.
Second for the long term ask yourself several questions. How much do you care about the bishop pair. Will trading the pieces muck up your opponents pawn structure (especially in front of the castled king)? Is the knight on the castled side and will it stay gone if you take it?
There are surely some other considerations but theses should be a good start.
http://www.chess.com/livechess/game?id=771518860
In this game I feel like I did a decent job with my opening but on the 10th move I moved Na5 trying to Fork his Queen and Bishop which I realize was a mistake. When he went Qa4 should I have just retracted the previous move instead of going b5?
Also how do you determine whether to trade your bishop for his knight when you pin it on let's say c3 and you are on b4. Whenever I go for that type of pin I usually find they move their rook side peon up one square and I'm not sure what to do. When I move it back a square they usually continue pressuring it by moving the peon in front of their knight up 2 squares.