Is the definition of "sin" subjective? Of whom's morals is it based on? Whenever I try to determine what is a sin and what isn't I get very confused... I read things by Paul and sins seem to become very subjective... 

_____ is a sin if you make it a sin through belief that it isn't acceptable due to your obeying of the law (the old law, before Jesus came)

then I read the old testament which makes it much more objective...

_____ is a sin because it is immoral (Referring to God's morals)

If the first is true than couldn't anything be justified as not-sin simply by a choice in a state of mind and what you believe your morals are (if that's the case why would you need Jesus for salvation, you are already free of "sin"... so that obviously isn't the case)

If the second is true than how do we ever know what is sin and what isn't? (The spirit that is given to us.)

Okay, but what about before the spirit came (personally and historically), how could anyone interpret wether or not they are sinning? I know the law was given but surely the law couldn't cover everything?

Is it all just a matter of the heart? I read about "sexual immorality" but what is that even referring to, immoral things in your conscience and the culture you live in or immoral as a definition of a standard of God that was never given to man... I see people who look at everything sexual as a biological process so of course they never feel guilt or remorse and it would seem almost unjust to classify their actions as sin. I see others who see it as something where ____ is culturally acceptable but ____ is not and is therefor sin.

How does one define sin? I know it seems like a broad question but I can't simplify it any further than what I've typed above.
Sin is absolutely objective but in its application in our lives has a subjective element.  The particular case that you mention is where Paul is saying if a person thinks something is a sin (even though it is not) then it is a sin to that person if they run about doing what their conscience condemns.  This is because anything done without faith is a sin:


  But whoever has doubts is condemned if he eats, because the eating is not from faith. For whatever does not proceed from faith is sin. (Romans 14:23, ESV)


In other words if you truly thought that wearing a purple shirt was a sin, you should not wear a purple shirt.

However the reverse is not true. Anything that is objectively a sin, like blasphemy, murder, sexual perversion, etc. even if you think it is not a sin, is still a sin. Sin is anything that is done not derived from pure love to God. This means we all sin daily as none perfectly loves God.

To answer your question: sin is objective (it is not loving God perfectly) but one objective sin is doing things you incorrectly consider as sin due to want of knowledge or wisdom. This is a sin because it shows lack of respect to God when you ignore your own conscience, even when it is confused on neutral matters.