I am writing a book where I have a main character who comes to a young civilization. The young society only works through barter trade, and he wants to introduce a form of currency, such as money. What is the best way to achieve this?
Basically you are just going to use commodity money, some good that is easily fractionated/transportable does not spoil and is universally recognized as valuable. This is the money used by the majority of the world for most of history.
In history the most common is cowry shells although weight of gold and other precious metals has also been popular if harder to measure. Basically you just have something everyone uses as for exchange, "I'll give you 30 cowey for that goat, and you can go trade 1 cowery for a few loaves of bread or a new shirt or whatever. This is the money used by the majority of the world for most of history.
You have a lot of options for a commodity, the tech level of your society will determine what is available, cowrie shells are easy to carry and impossible to counterfeit but get bulky if you need a lot of them, precious metals an easy to fraction but possible to counterfeit and have issues with assessing purity, measures of grain or salt are easy but get bulky in large amounts, bronze knives, liquor, animal pelts, and a whole slew of other things have been used, and of course you are not restricted to only one commodity although it is rare for more than 3-4 commodities to see usage as money together.
If your interested extra history has a great 6 part series on the history of money.
Now if you only have to deal with a single settlement there is one other thing you can do. Most large societies have communal grain stores, a community can build a grain silo but it is hard for an individual to do. governments would keep records of how much grain you submitted and give you a writ or token representing how much you contributed which you could trade with others. This does not work well if there are multiple nearby countries however, or once you get larger than a single town.
Fred and Bill own land nearby one another. Fred is a carpenter and Bill is a cow farmer.
One day, Fred runs out of food. He hasn't made a sale in a while, and his wood is all in poor condition because of a recent fire that messed up his storehouses. Bill doesn't have any need for carpentry work at the moment, but Bill needs some more land, because he has lots of cows. Bill and Fred make a deal.
Fred exchanges some of his land for food. But land is costly, and in order to exchange enough of it to make it worth Bill's while, he has to exchange the equivalent land-value for a lot of cow meat. Fred has no way to store that much meat, and expects much of it to spoil. Furthermore, having thus sold his land, he'd have no place to put live cows if he got them. So Fred exchanges several acres of his land for a promissory note, worth the meat from 100 cows, which Bill will care for in the meantime.
Later on, Fred decides to go buy some wood from Mr. Chuck who owns a lumber yard. Fred has nothing to offer at the moment, because he just gave up most of his land, and all his wood is burned up. He offers 50-cows of his promissory cow-note for some freshly cut trees to get his business back up and running.
Now Bill owes cows to both Fred and Chuck, and seeing as Chuck is a vegetarian, he shortly divides up his part of the cow-note in several exchanges with other friends. Before long, people are buying small items for notes worth less than 1 of Bill's cows. A new chair might be worth 1/16 of one of Bill's cows. Thus, a currency is born on the cow-standard, and the notes are shortly nicknamed "Bills".
The point is, currency isn't something you have to work hard to bring into a society. It's an inevitable consequence of the growth of society. Eventually, someone who only produces expensive items will need to purchase something inexpensive, and he'll either have to carry back lots of useless inexpensive junk, or just exchange his expensive produce for one promissory note.
I don't know of this is the best way to go about things, but I think I'd probably start with something like:
1. Personal contracts. Hey Antony, thanks for the haircut, and I know I owe you a chicken now, but I didn't feel like carrying a chicken all the way over to your barber shop, so instead let me give you this piece of paper: "I, Roger, agree to give Antony ONE (1) chicken, upon demand, when presented with this document."
2. Bearer bonds. From there it's hopefully a small hop over to: "I, Roger, agree to give anyone presenting this document ONE (1) chicken."
At this point we're basically already at currency -- we've got a private, chicken-based currency going. Good old cluckcoin. But we can push it a bit further:
3. The government horns in. The government is concerned that all this private currency is floating around, so they decide to establish a monopoly on it. They set up government booths where any citizen can exchange chickens for official minted cluckbucks, and vice versa.
4. The government makes it mandatory. At some point the government decides that the citizenry can no longer pay their taxes in annual chickens, but will only accept cluckcoin.
At some point they may decide to depart from the Chicken Standard (the deflationary pressures are likely to be a problem) but they might not; it might be out of scope for your story in any case.
Anyway that's probably enough to get the coin rolling.