Origins of Paladins
Paladins are holy warriors. The operative words being holy, as in divinely inspired or patronized, and warrior, which implies an amount of martial prowess in terms of being used against other living, and unliving things. All paladins must have a patron deity, and for good reason, without it they'd lose the majority of their class abilities, and they'd have no idea how to live their lives. Paladins are required to have a code. They must live by a set of principles or ethics to maintain their class. If you want to arbitrarily decide how to do good, and do it without 'restrictions', the chaotic equivalent of the paladin is the Holy Liberator. Amia doesn't have those.
For information on paladin orders for various deities, see this link.
Becoming a Paladin
Becoming a paladin is always a conscious decision. However, this does not imply that one simply bucks up and joins the paladin core, and becomes one in a brief training montage. Some people can be paladins, and some people just flat-out can't. About one in every thousand members of any sentient species has the capacity for being an adventurer, about one in every five hundred of those could become a paladin. Scale those ratios down extremely for inherently evil races. When one realizes they should be spending their life helping people and keeping things peaceful to the best of their ability, they go discover how to become a paladin. It's a calling, not an organization.
Paladin-hood is something culturally notable. People are not paladins because they feel like it, and they do not become paladins because they spent weeks in the wilderness communing with their god. Unless that's part of the initiation, but that comes later. Paladins are recognized by the priesthood, and generally welcome, though not all paladins work directly with priests. And, in some cases, the conflicting interests of the lawful good-aligned paladins and the slightly more flexible priesthood can cause them to not abide each other. Faiths fight within themselves as well as outside, even the good ones.
Paladins, as mentioned before, must have a code. The code is usually inherited from preceding paladins, which eventually extrapolates into what is known as an "order" of paladins. Most paladins have had at some point a mentor. People do not instinctively know how to do the right thing in all situations, and so, upon learning that there exist people with their same shameless drive for moral action in all things, paladins go to learn from them. On occasion this happens later in life, when people have encountered many paladins, until they find an order or mentor that best speaks to them.
The actual initiation, and when the character is deemed 'paladin' is decided either by the mentor, order, or society. The mentor may stand one day and say "You're done." to their pupil, call them a paladin and equal, and send them packing. Equally, an order may set forward a specific set of guidelines or test to finalize inclusion. Less frequently, upon the death of mentor, or unavailability of the order, a paladin my deem themselves to be 'paladin'. This is an extremely rare situation, however. And, as always, the final decision is up to the god.
Being a Paladin
Any character, ordained and armed as a paladin, generally armored, is suddenly put in a precarious situation. Unlike most priests that can say "I am a cleric of Cyric." and have the common folk flinch, paladins are not recognized as frequently by their faiths so much as their occupation. A paladin is a paladin to most people. This means, among other things, that people do not come to paladins for consolation, answers to life's questions, or donations. They do, however, hear 'paladin', and think of evil that they need squelched. Paladins are frequently heroic figures, and often more than any other class manage to rise into higher levels of power. The common people will see a glowing, peerless fighter who can defeat dragons with the loss of only an eyebrow, regardless of how powerful the paladin actually is. It is for this reason that paladins are frequently roamers. It's unhealthy for people to expect their heroes to be on-call for the rest of eternity, and having the convenient and completely true excuse of "I must move on." allows a paladin to occasionally delegate problems beyond him or her to someone more capable.
Paladins, by and large, are not idiots. They tend to be well-educated, civilized and highly socially aware. The majority are astute, considerate and fairly blunt. They are, more than normal people, aware of what their action or inaction is doing to the world. Should they linger for the grand feast when that ring really needs to be thrown into the caldera? Probably not. They know this.
Yes, it's possible and completely understandable to get the occasional naive paladin, but they do not tend to live long or at all. Quixote should have been killed three times over, if he had not somehow chosen highly inept people to 'challenge'.
What the paladin must do in their life, however, is adhere to the tenants of their code, and do good. You must, and may not ever willingly commit an evil act for the sake of a sin. There are some murky waters surrounding this general statement, but it remains by and large inviolable. If the shopkeep is refusing to tell you where the evil ritual to bring ruin to the world is taking place out of fear of reprisal, you have permission to break his nose to get the information out of him. You're holy warriors, darn it. Not holy wusses.
That being said, a paladin is not to be faulted for on occasion not showing compassion, or getting fed up with people. They are not to be faulted for raising a fist in anger, briefly, given that they lower it with significantly less anger.
The largest development point for a paladin is probably selection of a deity. Lawful good and neutral good deities really never conflict with their paladins. They say 'go for it'. No issue with a bit of law and good in the world, even the neutral ones will concede that point. The god of a paladin defines their priorities. A paladin of Tyr wants justice out of all situations, whereas a paladin of Kelemvor or Illmater would likely be interested in less life being lost in any situation. Always take your deity's preferences into account before acting in a given situation, as a paladin.
There is a lot of variation and development for different ways for people to play paladins. They are not all the 'crusader' type, nor the righteous avenger. They have as much variation in preferences and activities as rogues or fighters. Just keep in mind the best intentions and a slightly stoic mindset concerning life, and you should be good.